The doctor will see you now: Who does Vernon Coleman think he is?

You'd be forgiven for thinking that Dr Vernon Coleman is an oddball, a weirdo, a bona fide, card-carrying English eccentric. After all, this former GP, TV doctor and newspaper columnist-turned self-publishing phenomenon has spent the best part of 40 years kicking against authority, speaking his mind and generally pissing people off.

According to his website, the good doctor has shifted more than two million copies of his 90 self-published works. Among the most notable of his non-fiction offerings are Gordon is a Moron, How to Stop Your Doctor from Killing You and How to Protect and Preserve Your Freedom, Identity and Privacy. If these don't whet your appetite, you could always delve into his fiction list, which includes the titles Mrs Caldicot's Cabbage War ("Now a major film starring Pauline Collins", says the cover blurb) and Alice's Diary, which is written from the viewpoint of a cat.

He has agreed to be interviewed about his newest publication, Oil Apocalypse, in which he turns his attention to the imminent end of the world, which he foresees as a direct result of the looming global oil shortage. There's no getting around the fact that this is a very bleak book.

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"The world you know is going to change dramatically and permanently," it says. "Anyone under 50, with a normal life expectation, will live to see a world almost unrecognisable from the one they grew up in. Five billion people will die within a very short time." And that's just on the cover. There are 153 doom-laden pages nestling inside.

Vernon Coleman is a vegan and came out, in 1995, as a transvestite. He said cross-dressing helped him to relax and find his softer side. "I don't become so upset and cross about things," he has said. Of course, these days, thanks to those national treasures Grayson Perry and Eddie Izzard, transvestism is slightly less strange than it used to be, but still, I'm feeling a slight air of trepidation on my way to our meeting.

If everything I've read is on the money, I fear I'm about to be greeted by a ranting maniac in full drag.

So when I arrive at the National Liberal Club in Whitehall I'm surprised to find a tall, slim gentleman in a beige suit, a blue button-down shirt and a bacon-and-egg tie, spectacles around his neck. His shoes are pure GP: sensible large brown leather things with stout laces and thick rubber soles. The only clue to his transvestism is the ladies' engagement ring he wears along with his wedding ring. He has an uncontrolled thatch of grey hair, pale blue eyes and a large nose, which makes him look a bit like the clockwork woodpecker, Professor Yaffle, from the children's puppet series Bagpuss.

He speaks so quietly that I can hardly hear him, and he inquires solicitously, as I arrive coughing and sneezing, if I am suffering very much from hayfever this year. "All the consolation I can give you is that it gets better with age," he says kindly, as we find a quiet corner in which to sit. (I half expect him to say: "So, what seems to be the trouble?")

Soon, he's in full flow, turning swiftly to literary matters. He enjoyed Evelyn Waugh and Graham Greene as a child, and his favourite was PG Wodehouse. "People sometimes turn their noses up at PG," he says. "But for me, he's the greatest stylist there's ever been, his use of words is stunning."

Can this really be the man who irritates so many people? Who causes his enemies to spread rumours that he never even went to medical school? Whose prose style is so argumentative, whose opinions are so trenchant?

Perhaps it is age that has mellowed him. He is 61, and gave up his famous agony uncle column in The People in 2003, after the editor declined to publish some of his comments about the Iraq war. Coleman said that people who opposed it shouldn't simply be dismissed as peaceniks and hippies; the editor thought this unfair to "our boys and girls in the desert". The column ran for 11 years and was as racy as tabloid medical columns tend to be. One of his typical problems might have been: "My boyfriend never satisfies me. He can rarely make love for more than an hour, and that simply isn't long enough for me. Is there anything we can use to make things last longer?" But in its latter years, the column broadened out to discussion of other issues, such as hospital bugs and the suggestion that the police might be sued when there is bad traffic. "Most policemen are loaded, thanks to huge salaries, fat pensions and large compensation payments," wrote Dr Coleman, memorably. "They will make rich pickings for aggrieved motorists..."

Since his departure from The People, Dr Coleman has withdrawn to his Devon home, where he knocks out his bestsellers at an impressive rate. He must work all the hours God sends to keep the word count up, but he is reluctant to discuss his routine. "It's quite difficult to know how much you work, when you're a writer, because you never stop." He writes on a computer, using the ancient writing program WordPerfect 5.1 ("I'm probably the only person on the planet who still uses it"). He has a mobile, but it is always switched off, and he rarely uses email, although he did, until last week, own a BlackBerry. "Hardly anyone had the email address." Until it broke, he used it to check the cricket scores and to find out "who was that actress who appeared in such-and-such a film".

He sells his books via mail order from his website and places huge, intricately worded advertisements in newspapers, including The Independent. His backlist is so huge that it has to be divided up in to "Animals, cats, humour, medical, novels, the Bilbury series, politics and other stuff, sport (fiction)". (The current bestseller list is topped by Oil Apocalypse, followed by Gordon is a Moron, How to Stop Your Doctor Killing You and The Truth They Don't Tell You (And Don't Want You to Know) about the EU.

"A [book] title should tell you what the book's about, but it should also be catchy," he says. "I've always liked good titles. I went to a Portuguese publisher and they asked me what I had done. They looked at How To Stop Your Doctor Killing You and the publisher said, 'that is the first book I want to publish'. I asked if he wanted to see a copy and he said 'I don't want to see it – I just want to publish it.'"

It may seem that Dr Coleman churns out books on an endless variety of subjects, but in fact, he observes, they have a common theme. "Freedom and independence. The novels are often about various forms of escapism. I've got a series of novels about a village I made up called Bilbury, in which the villagers have to protect themselves against all sorts of bureaucracy; they just want to be left alone."

Another Coleman bestseller, Mrs Caldicot's Cabbage War, captures the spirit of his work. He wanted to write something about the shabby treatment of the elderly in nursing homes, but decided that that would be "boring", so he wrote a story about a woman who is put into a home by her children (who want to sell her house) and she leads a revolt among the inmates.

"I just write stuff I want to write: stories I want to write and non-fiction books that I think are important – on subjects where I think someone else isn't saying something that I think needs to be said."

Dr Coleman lives in Barnstaple with his wife, Donna Antoinette, whom he describes as "the Welsh princess". He met her because she wrote to him via his Sunday People column; he wrote back and it went from there. Following his highly publicised anti-vivisection campaigning – which saw him banned from mentioning in public the name of Professor Colin Blakemore (former chief executive of the Medical Research Council and defender of vivisection) after Coleman threatened to publish Blakemore's address – he claimed to have received "threats" and moved to Devon, where he guards his privacy jealously. He reveals only that he has "an address in London and an address in France". Where in France? I ask. "In the big city," he says, reluctantly. "In Paris." He and Donna Antoinette go there when they feel like it; Coleman has set four of his novels in Paris, but still, to his chagrin, can't speak the language. "The French are nice when you attempt to speak the language but they always correct your pronunciation. If you ask for a Pernod the waiter will say "Actually, it's Per-noh," but then another waiter will say, "Actually, it's Per-noe."

Coleman was born in 1946 and grew up in Walsall as an only child. His father was an electrical engineer, his mother a housewife. "She didn't do anything except run the house and look after me, which was probably pretty hard work, until she was about 45," says Coleman. "Then she started her own company, doing stationery and printing, and then she bought a haberdashery shop. She was a little whirl, bless her."

At Queen Mary's Grammar School for Boys in Walsall, he wasn't good at anything in particular. He just remembers spending all his time in the school library "reading all the books, all the novels". He didn't really know what he wanted to do and, being at a grammar school, thought the only choice was between law and medicine. "Then I met a friend of the family when I was about 12 who said, if you're a lawyer you spend your life making people unhappy, and if you're a doctor you spend your life trying to make people happy."

Before he went to medical school, Coleman decided that he wanted to see a bit of real life. He worked as a volunteer in Kirkby, Liverpool, getting kids to paint old people's houses and to do their shopping. "The council didn't like it and the unions hated it and threatened to strike because they were taking away work – but work that they weren't doing anyway. So the kids loved it because they were doing something that the council was against, and everyone else was against it, so it became quite successful. I got the paint from a paint factory and moved it around in a Meals-on-Wheels van."

It was that experience, Coleman says, that turned him into the free spirit he has been ever since.

He was a GP in Leamington Spa for a decade or so, but he grew sick of form-filling and was fined for refusing to write, on sick notes, what was wrong with the patient. "One patient came to me asking me not to write what was wrong with him because otherwise he wouldn't get a promotion, so I didn't."

After the fine came his first book, The Medicine Men, which accused the NHS of being in thrall to drugs companies. It was serialised in The Guardian and after that, it was all over; from then on he stood no chance of being respectable in the eyes of the medical establishment. But he "didn't give a toss" about leaving general practice. "I wanted to write."

He began writing for newspapers in 1964, and has written for, among others, the Daily Star and The Sun, which he describes as being "exhausting". He would, frequently, have only 30 minutes to write 750 words on what it would be like to die in an aeroplane crash, or what it would be like to be in the front row at the Hillsborough disaster. Phew.

But it was his column in The People that really made his name. Slots on TV-AM followed, as well as premium-rate "advice" lines.

The self-publishing came later, when he wrote a book from the viewpoint of his tortoiseshell cat, Alice, now, to his great regret, deceased. "All these publishers said, 'who's going to buy a book written by a cat?' But I liked it: I thought it was good fun. I'd also written another non-fiction book called Betrayal of Trust and no one would publish that because of libel laws, so I thought, bugger it, I'll publish it myself." Surely it must have been expensive to do that? "Not really. And Betrayal of Trust sold 6,000 hardback and I've lost count of the number of times that Alice's Diary has been reprinted. It was 50,000 [copies] years ago. I just print 5,000 at a time every now and again." So there must be a market for a book about a talking cat.

"Every book I've ever written, I've written with passion, without thinking whether it will sell or not. It's not a very sensible way of doing things. Not at all, it's really stupid." He hasn't suffered from it, though; at any one time he has 50,000 books in stock.

He has been married to Donna Antoinette for nine years and was previously married, for eight years, to a nurse while he was a practising GP. Yet he has never had children. Why not?

"It's very complicated," he says, and then pauses for a long time. "Largely, it's because I'm very pessimistic about the state of the world and its long-term future. I wrote a book, called Why Everything is Going to Get Worse Before it Gets Better. It was originally going to be just "Why Everything is Going to Get Worse", but that sounded too gloomy. But I do think people have lost it, to be perfectly honest. Politicians have forgotten what they are for and people have lost interest, so politicians have more power and less responsibility, it's sad, really."

And so far as oil is concerned: "I think [nuclear power] is the only answer, what else are you going to use? As long as you're sensible, it causes far less deaths than something like coal mining that kills thousands, and everyone is very happy with that. Or oil rigs. If you don't use nuclear power, what are you going to use? We'll have rioting in the streets. I'd rather have nuclear power than rioting in the streets and mass death, which there will be. Until you start thinking about it, you don't realise how many things that oil is used for. We've built a society on a freebie. Once it starts to run out, you get to the point where you've got less, just as the world starts wanting more. It's just an enormous mess."

He denies, however, that he is a scare-monger. "I'm actually anti-scaremongering. I've been around a very long time," he says. "A lot of people who buy my books are used to the fact that I don't scaremonger for the sake of it, it's like not crying wolf."

Our time is up and Dr Coleman has to dash to his next appointment. As we part, he tells me that he still reads all the time, and never returns to the house without more books than he can carry. Before going back to his secret hideaway in North Devon, to Donna Antoinette, his WordPerfect 5.1 and his dressing-up box, he will visit the small independent bookshops of Cecil Court, where you can find obscure, out-of-print titles. He admits to buying some books on Amazon, but believes the web behemoth is going to destroy authors. "You can buy almost any author on Amazon for a penny." It's an interesting theory – you never know, there might even be a book in it.