Paul McCartney, rock star, family man, northern lad, contender for most famous person on the planet, is leaning back on a squashy sofa. In just a short time, he will announce that he is not going to lunge forward and kill me. However, for now, he's musing on his favourite-ever vegetarian ingredients.

"Could I have a few?" he asks eventually. You can have as many as you like, you're Paul McCartney. "OK, so… olive oil, balsamic. There's this great hummus you only seem to be able to get here, 'Amvrosia' – Ambrosia with a v, I use that a lot. Lemon juice, salt, spinach leaves, rocket leaves, plum tomatoes. You see, it's getting good already."

You're a decent cook then?

"I'm not bad. I can turn a meal out."

McCartney loves steamed vegetables. "If I go on tour and eat a lot of restaurant or hotel food, I come back, and it's like, yeah, broccoli! So, if I'm cooking, I'll be steaming vegetables, making some nice salad, that kind of stuff."

Do you follow recipes?

"No, I just make it up, like Linda did."

We meet at the wonderfully named Hog Hill Mill, McCartney's private recording studio in Sussex. As so often happens with extraordinarily famous people, there is that initial disbelieving lurch when he's there standing in front you – famously youthful face, slightly boppy "musical" movement to his walk, 20 years dropping away when he smiles.

The upstairs room where we talk is full of memorabilia from all points of his 50-odd year career. Just before our meeting, McCartney went to the White House to receive the Gershwin award for popular music, and sang "Michelle" for the Obamas, which he describes as a career high. "Obama leaned over and was singing it to her. Someone later said they were like a couple of teenagers."

At just turned 68, McCartney seems like a bundle of energy himself. After our interview, he will return downstairs to the studio to play with a new computer programming system for music. He's not anti-technology then? "Not really, but this is the only time I'd sit with my head in a computer for six hours." Are you the type who gets bored when they don't work? McCartney thinks for a moment. "Probably."

We're here to talk about Meat Free Monday – an idea McCartney first heard about in America, which he feels ties in with the UN's 2006 Livestock's Long Shadow report, which outlined how the livestock industry was responsible for more greenhouse gases than transport. "Though the UN, who are presumably carnivore, went further – they said, the best thing you can do for the planet is go vegetarian," says McCartney.

He felt that one day a week was "do-able". "If you ask people to go completely vegetarian, it may be too challenging. This wasn't asking so much."

What would he say to those who argue that one day a week isn't enough? "I think any move in the right direction is enough. If you want my real point of view then, yeah, I would encourage people to go vegetarian, but that's not what this campaign is about. And it's not what the UN campaign was about."

McCartney says he has a lot of friends who eat meat, and likens it to religion. "I wouldn't want someone to be bossing me, saying, 'You should be a Buddhist.' I'd say, 'Lay off, I'll make my own mind up, thank you!' Similarly, I don't want to go laying it on people – 'You really should be vegetarian.' I like them to come to it themselves."

What food did McCartney enjoy BV (Before Vegetarianism)?

"Just normal restaurant food really. I'd go to America and have the biggest steaks in the world. So big I'd have to give half of it away. Or I'd have chicken kiev with all the butter oozing out of it. That was great."

Of his own conversion in the mid-70s, McCartney has often told the story of how he and his first wife, Linda, were having a roast dinner, looked out of the window, saw lambs gambolling, and realised they didn't want to eat meat any more. "It was like, the penny dropped. The light bulb lit up. We thought, we might just give this up."

It's safe to conclude that Linda is the key to McCartney's vegetarianism, indeed that of the whole McCartney clan. Before her death in 1998, she had huge success with her series of meat-free cookbooks, and eponymous food range. Meat Free Monday sounds very "Linda". Is she still McCartney's main inspiration?

"Yeah, I'd say so. She was the original inspiration and she had a way about her. There was just this… non-aggressive forcefulness, and many of our friends over the years became vegetarian because of that."

As a vegetarian, I have a rather more prosaic admiration for Linda McCartney – she probably stopped people like me (lazy vegetarians) starving; definitely from having to learn to cook in any meaningful way. When her food range came out, it seemed to kick-start a revolution of choice in the food industry: suddenly there were a lot more vegetarian products available.

"I think you're right," says McCartney. "Back then, we dreamed that one day you would pull off a motorway and there would be vegetarian options. We're there now and that's great. With the range, we used to call it 'the hole in the middle of the plate'. Where you'd have meat chops and vegetables, steak and chips, we only had the 'and', we didn't have the centrepiece of the meal. So we had to think about how to do it.

"The really lovely thing about it was that people from Linda's food company would say, 'Oh, Quorn is coming out with a burger', and she would say, 'Great!' Which was fabulous. She was not the typical businesswoman. Instead of [snarling corporate accent] 'We've got to kill their burger! Ours must be the only one!' she wasn't like that. Her attitude was: the more the merrier."

What about bringing up their family as vegetarian: didn't any of the kids rebel – even a little bit?

"With vegetarianism?" says McCartney. "No, we were very lucky. There was one moment on holiday when the kids were quite young. And they said, 'Can we have a chicken dinner?' And we said, 'You can, but you know it's those things we have at home, the little chickens you love so much.' And they said, 'We still want to try it.' I said, 'Yeah, OK.' So they did, and they didn't like it."

Of course McCartney isn't just any old vegetarian – he has global influence. He tells me a story about finding out that the Dalai Lama wasn't vegetarian, and writing to ask him about it.

"Dear Dalai Lama"?

"Yeah, Dal to his friends," he says laconically. "He wrote back very kindly, saying, my doctors tell me that I must eat meat. And I wrote back again, saying, you know, I don't think that's right. So we had a little correspondence."

How did you leave it with him?

"I think now he's vegetarian most of the time. I think he's now being told, the more he meets doctors from the west, that he can get his protein somewhere else. It's a little old-fashioned to think that he can only get it from meat."

McCartney is happy to acknowledge that he wields the kind of clout that means he can embark on a personal correspondence with the Dalai Lama. "But even if I hadn't been 'Paul McCartney', I'd have written that letter. It just doesn't seem right – the Dalai Lama, on the one hand, saying, 'Hey guys, don't harm sentient beings… Oh, and by the way, I'm having a steak.'"

Despite the enlightened atmosphere of the 60s and 70s, McCartney thinks attitudes towards vegetarianism are generally better these days. "People have got over all sorts of old prejudices, haven't they? Things have progressed. I like it, it's more civilised."

What about male vegetarians – doesn't McCartney think they get a rough deal?

"What, you mean – [droll] 'real men don't eat quiche?'"

I tell McCartney my theory about him and male vegetarians – that they're often ridiculed as unmanly, and he's the only thing that makes their lives bearable. He's a vegetarian guy they can point to, who is this world famous rock icon.

"Hmm, I don't know," says McCartney. "But you know, as a kid I would have thought of a vegetarian as a wimp. I would have automatically laughed at them, gays as well. Coming from Liverpool, we were a pretty prejudiced little bunch. Believe me, there was no one who escaped our scathing comments. But then as you grow up you think, well that's pretty childish and daft. But of course it would be, you were young. What you're going to do now is grow.

"So, while I feel perfectly manly [he stresses the word, with a slight roll of the eyes] eating vegetarian, and it doesn't seem that way to me, I can understand the people who still feel that way. Because it's how I felt when I was a young idiot. Maybe they're still at that stage, you know?"

There still seems to be this lingering perception that vegetarianism is a "feminine" diet/mindset?

"It's funny that, isn't it?" muses McCartney. "The big bad guy thing. There's also the inevitable backlash. So, everyone is thinking about vegetarianism but… [growly macho voice] not Marco Pierre White!"

Gordon Ramsay is the one who seems really anti-vegetarian.

"Yes, but I hear Gordon is advocating one meat-free day a week. And that the new Jamie Oliver book is going to be veggie, and he's advocating two meat-free days a week. So it's happening, it's interesting."

Oliver, Heston Blumenthal and Hugh Fearnley-Whittingstall use meat in their recipes, but they've been prominent campaigners for better conditions for animals – what does McCartney think of that?

"I'm all for any movement in the right direction," he says. "I'm not black and white. You know, 'He's a baddie if he eats meat'. I'm not all or nothing. With those guys, as you say, the good husbandry is a big side of it. I would go further and say, well then, don't kill 'em, don't eat 'em, perhaps? But the fact they are considering not eating them, one or two days a week, is a move in the right direction."

Getting back to that poor "unmanly" vegetarian guy – what words of comfort could you give to him?

"Well, that super-macho, super-butch thing is all a bit silly and immature if you ask me," he says. "It reminds me of a lot of things I did as a kid, when I was running around with my catapult. We were a savage little lot, Liverpool kids, not pacifist or vegetarian or anything. But I feel I've gone beyond that, and that it was immature to be so prejudiced and believe in all the stereotypes. And anyway, this thing of a man must be a great big guy with a hairy chest who eats a lot of meat. Well, actually, medically no, you're going to be a very short-lived great big guy with a hairy chest who eats lots of meat – you might not live that long."

McCartney explains that, for him, vegetarianism came as an "epiphany".

"I just realised: I was taking animals' lives. And some people will laugh at that. But, for me, it's like when I gave up fishing, on a ranch in Nashville. We were working there and this guy had a lake. So I was fishing and enjoying it and I caught a fish and pulled him out and I could see the little thing struggling for his life. And whereas normally I would have said, 'Yeah, you're my dinner', and put him in my tin, I just thought, 'Oh, I'm really killing you, and I could do something about this.' So I gently got the hook out and it was, 'There you go mate.'"

You objected to the cruelty?

"Yeah, just the idea of it," McCartney glances over pointedly. "I wouldn't jump across the couch and take your life."

Thanks.

"Yeah, well, I don't want to make you nervous!" he laughs. "But you know what I mean? In our society, to take another person's life you have to be a bad person. I started to see that that logic extends into all of the animal world. That's how it feels for me anyway."

Some time later, I find myself backstage at McCartney's Hyde Park show. The idea is to see how the world of rock'n'roll takes to vegetarianism. Anyone who's ever seen what roadies normally eat (a kind of Desperate Dan cow pie cuisine), would realise that Macca's meat-free environment is quite an achievement. The fact that the food looks top-end and delicious is all credit to Eat Your Heart Out, McCartney's on-tour caterer.

I talk to the New Zealander chef, Neil Smith, who is busy chopping up a melon. He has cooked for Eric Clapton, Rod Stewart and Britney Spears. What does he feel are the special challenges of vegetarian food? "Just getting the nutritional balance right – protein, the right amount of fat and carbs." What does McCartney like? "Plain simple things – quiche, pasta, bean burgers, vegetarian cottage pie – he likes normal food done in a vegetarian way." That makes sense, I think, recalling McCartney's "hole in the middle of the plate".

Would veganism be tougher? "Oh yeah, no cheese to fall back on." Smith runs through some tour dishes – savoury crepes, salads, pastas, anything involving tomatoes, fennel, broad beans, potatoes, tofu, tempeh. Is there any grumbling about the enforced vegetarianism? "No. Most people have been working for Paul for a long time, they know that's how it is."

One of these long-time crew members, Kevin Smith, agrees. "There are usually some people at the beginning of a tour who'll say, 'I'm not eating that shit, I want real food.' But Paul said, from day one, 'Go buy your own burger, just don't bring it back here and eat it, have respect for what we do.'"

Smith became fully vegetarian after a year working with McCartney – previously he'd been "borderline", and then Linda spoke to him. "She was quite persuasive, bless her." He thinks, if anything, that backstage food is even more important to the crew than the band: "Band members live fairly mollycoddled existences. They're at a hotel, they have lunch whenever they like. Crew are here all day – food becomes incredibly important to you."

Earlier, I had asked McCartney, what does food do – nourish people, bond them?

"Well, it keeps them alive," he replied. "But no, seriously, food is really important for all that. Family meals are great. Linda and I always liked the idea of an Italian family [he launches into an Italian accent], Mamma mia! You know, with Italian momma cooking the meal, and Italian father getting the wine, I think all that is great."

One final question: does he think that he'd have been vegetarian if it hadn't been for Linda – that he'd have gone that route anyway?

"That's an interesting question," muses McCartney. "I don't know. What I do know is that the two of us together were more than the sum of our parts. We had a lot in common – which you'd hope if you're going to get married." He gives a slightly bashful laugh. "But yeah, when we got together that was one of the things we had in common – this huge love of animals. And, like Linda, I loved animals as a kid. Because even though I talk a lot about the savage youth, the youthful days in Liverpool – and there was all that – I also used to go out with my bird book, and I loved all that, too."

What would that young boy think of you now?

"Nutter!" says McCartney instantly. "He'd get his catapult out and aim it at me." He grins at the thought. "But you know, I would have to understand – charming little boy!"

for more on Linda McCartney Foods: lindamccartneyfoods.co.uk