Brett Kahr is a bright, polite, charming man whose early academic career took him from Cornell to Oxford to Yale; he also has more sexual fantasies in his head than anyone who has ever lived. Kahr, who is 46, has been collecting these fantasies on and off for years, but since 2002, he has considerably increased his productivity rate: 19,000 fantasies from the UK, another 4,000 from America. Each one has been carefully catalogued and archived: B for bondage or bestiality, C for coprophilia or Cherie Blair.

Kahr is a Freudian psychotherapist based in Hampstead, north London. When I meet him in his consulting rooms - there's a couch for lying on and a supply of tea and organic mango juice - he sits a little uncomfortably in one corner, not quite used to being the subject under discussion.

He is about to publish the findings of an ongoing obsession, Sex and the Psyche, the most comprehensive analysis of the secret desires of the British ever undertaken. I wonder a bit about the motivation for the book. He says he knows all the Woody Allen jokes, but he believes, too, that there was no getting away from it. Twenty-five years of listening to people expose their innermost fears and longings had convinced him that nothing was more urgent; sexual fantasy is at the heart of the anxieties of 'both healthy and tormented individuals,' he says. 'I could not,' he adds, with the doomy finality of a man accepting his fate, 'avoid the subject any longer.'

In his book, and in person, Kahr is acute about the oddness of a job which involves him starting work at 6.45 in the morning (many clients like to see him before they start work) and grappling all day with the details of interior lives that may promote 'thundering orgasm' or lead to suicidal despair. In his youth, he wanted to be a concert pianist. Instead, he spends his time working through the traumas of, say, 'Mrs Elphinstone' who, when she masturbates, thinks 'about her elder brother and her 17-year-old nephew, Claude, both of whom have really hirsute chests which she adores' - in contrast to the torso of 'Mr Elphinstone', which appears to her like that of 'a skinned chicken'. His book is full of such crippling dilemmas, so many that you get quickly inured to the peculiar sadness and comedy of human desire.

The bulk of respondents in Kahr's study, a representative 18,000, revealed their most intimate daydreams to an internet questionnaire organised by the pollsters YouGov. Much of his analysis, however, is based on 132 five-hour face-to-face interviews with volunteers from all walks of life who discussed with him their sexual biographies.

Kahr seems unfailingly interested in the extremes of British lust, though he insists that these are fantasies of absolutely normal citizens; 'none of them has been hospitalised or imprisoned; they are all at liberty,' he says, a thought which becomes increasingly alarming as his book goes on. He approaches each of these secret stories with undimmed curiosity, 'like an anthropologist who has stumbled upon a relatively untouched, faraway tribe'.

Some of the revelations must have tested his professional straight face. One man confesses to his long-held wish to 'bind both the Queen and Baroness Thatcher with ropes and then make love to each woman in turn'. Some are deeply disturbing - a woman whose parents perished in the Holocaust who has always become aroused by the thought of SS officers in jackboots. Kahr says he has been for a long time intrigued by a comment of Freud's suggesting 'that every sexual act is a process in which four persons are involved': the two people in bed and the two others in their heads. He sees himself in the tradition of Alfred Kinsey and Nancy Friday, a liberating force, and insists that that force is still needed when it comes to the British and sex.

'Kinsey in particular has been a huge influence,' Kahr suggests. 'He was an extremely brave man. I don't think it takes great courage to write a book about sex in 2007, but in 1948 it did. Before Kinsey, no one in America knew that anyone else masturbated. After his book came out, it turned out that pretty much everyone did. In that alone, he did humanity an incalculable service.'

Kahr believes he has taken Kinsey's 'huge platform' and tried to add one missing piece to it. He has tried to discover not what we do in the bedroom, but what goes on in our minds while we are doing it. He laces his confessionals with general conclusions and a compelling array of tables and extrapolated facts. The conclusions include the observations that the vast majority of British adults always fantasise about someone other than their long-term partner, that many fantasies involve forms of harm that, if acted on, would put their authors in prison, and that a surprisingly small percentage of people fantasise about celebrities, but when they do, they are often abusive.

Along the way, he charts some of the patterns of British sexual behaviour: the fact that eight million adults in Britain are sexually inactive, while half a million have sex more than once a day; or that the average British heterosexual male has 15.64 partners in a lifetime, while the average British woman has 14.56 men (these figures are significantly skewed, he adds, by a very promiscuous 4 per cent, 1.8 million British adults, who have had more than 100 partners). Three-quarters of all adults, male and female, admit to 'current masturbation' (though presumably they were able to break off long enough to fill in the form).

The internet is increasingly the stimulus of this solo pursuit: more than half of Kahr's male respondents admitted to having surfed the web for pornography. There is a strong taste for the home-grown: the British find Cliff Richard more of a fantasy figure than George Clooney; Carol Vorderman comes out ahead of Nicole Kidman. Some facts suggest a geographical variant: Scots of both sexes have far more success in reaching orgasm than Londoners.

These facts are punctuated in Kahr's book by a couple of thousand quoted fantasies in no particular order. Some cut to the chase: 'Being on a boat with loads of big-breasted women'; 'Shagging my secretary in the stationery cupboard' or a 'football changing room!!!'; many are sad: 'Sex with my husband like it was before we were married'; 'Stu from Neighbours'; 'Philip Schofield'. Many are so extraordinarily elaborate, involving unexpected encounters in copses with all sorts of well-endowed family members and office juniors, that it is exhausting to keep up. When he speaks of his project, Kahr tends to characterise his progress in it as 'wading through' or 'mired in' or 'immersed in a vat of'; there are times, I imagine, when only his good humour kept him from going under. He had to offer counselling to the small army of typists which transcribed his tapes; his hard drive exploded at one point and he could not help but think it was because of what he had exposed it to.

It's odd, but when faced with 556 pages of sexual fantasy, you quickly find yourself flicking past the apparently repetitive desire of the British male to engage with his sister-in-law bent over a kitchen table and instead dwell on Kahr's analysis. In many ways, his interpretations leave you wanting more. Having presented the often hilarious, frequently tragic spectrum of British desire, he is scrupulously equivocal about the implications of it all. Sometimes, fantasy is entirely healthy; sometimes, it is totally destructive. Sometimes, it is a good idea to tell your partner 'all your heart'; often a fantasy shared is a relationship finished. In the absence of a prescriptive relationship with desire, Kahr makes the argument that, at the very least, his study will help the average reader understand that he or she is not alone in interior weirdness, that perversion is a relative concept, that we are all in it together.

When it comes to discussing his own sexual biography, he is, understandably, a closed book, not wanting the details of his life to get in the way of his ongoing relationships with his patients, 'all of whom are Observer readers', some of whom have been consulting him weekly for 10 or 15 years. He lets a few details slip: he grew up in New York with Freud on every bookshelf; his mother is an art historian, his father was a property developer. His vocation was in the blood though. Three generations back, his family was Viennese and there was a great uncle who trained with Freud in the 1930s. He is, he grudgingly admits, 'partnered with a woman, no children'.

I wonder whether he thinks, during his time practising in London, that Britain has become more sexualised and more open to Freudian analysis? He approaches this question, like all my questions, with some careful gusto. 'Well,' he says, 'when I started working in this field in the early 1980s and told people what I did, they would recoil and say, "God, I bet you are analysing me and reading my mind right now!" A paranoid reaction. In the Nineties, it was so different: people would say, "Gosh that is so fascinating, I would love to find the time to see what was going on in my head." Now in 2007, people are so sophisticated when I mention it they will start comparing Freudians they have seen with cognitive behavourists. The stigma, you might say, has faded greatly.'

He thinks programmes such as Sex and the City have helped us to be able to talk about sex and to laugh about it, which is always 'a great developmental achievement - though obviously people who have grotesque abuse as part of their sexual history are going to find that extremely difficult'.

I wonder how much censorship there was of fantasies that appeared in the book? 'Well,' he says, 'the most horrific fantasies do not come anywhere near getting into print. I did not feel comfortable putting in the extreme murderous fantasies and the extreme fantasies around the abuse of infants and children.' These made up perhaps between 3 and 4 per cent of the whole sample; it was a great frustration of the internet survey, he suggests, not to be able to offer professional help to some of the most troubled respondents. The more 'gratuitously graphic in terms of blood lust' have been archived, however. Kahr hopes he can find a permanent home for his paper mountain of desire at a new institute of clinical sexology, available to others working in the field.

One of the things he hopes he has brought to that discipline is the insistence that these things matter greatly. 'The big criticism of British psychoanalysis worldwide is that it has neglected the sexual element and looked much more at mother-infant bonds, attachment theory,' he says. He recalls being told when he trained in couples therapy at the Tavistock Clinic that an analyst should not ask a couple about sex unless the couple brought it up first. He couldn't help believing that this was wrong. Experience has led him to suggest that contentment in the bedroom is often the subtlest barometer of the solidity of a relationship.

I wonder, conversely, if he has any truck with the old-fashioned British notion that the more we focus on our own fantasies, the more self-obsessed we become, the more subject we are to them.

'Ah!' he says, caricaturing this position, 'the "just get on with it" school of thought.' He laughs. 'It's a nice idea. But let's take the case of a man who is obsessed with his sister-in-law, a trope which I have seen countless times, a displaced incestuous anxiety. These men desperately try to force that idea out of their mind; it's like the Woody Allen voiceover always going on in their heads - "What am I thinking? She's got kids! She's my wife's sister!" - but in fact the thoughts in these cases do not disappear; in my experience, they become intensified. Most people do not have the capacity to block their psychic life. The only way they can do that is by using external things, whether it is prostitutes, pornography, alcohol, smoking, drugs, whatever, to medicate the mind. These are not healthy options. And, unmedicated, these thoughts will exacerbate...'

In many ways, though, Kahr sees fantasy lives as a way out of some of these psychological dead-ends, a mechanism that allows us to indulge our darkest wants without necessarily acting upon them. He suggests, too, that generationally we may be becoming more comfortable with the idea of fantasy. 'My editor at Penguin is in her twenties,' he says. 'I think she thinks a lot of material in the book is quite tame. Most people I have spoken to of her age are completely unfazed. But the oldest slice of the population will possibly be horrified.'

Does he think the MySpace and blogging generation are so used to sharing intimacies in an anonymous environment that it is becoming second-nature to them? 'I think we are certainly entering a new era,' he says. 'It may be to kids at university I will be seen as part of a Mary Whitehouse generation, being prudish about cybersex. I am not a computer man at all; I feel I write best with a pencil. But certainly all of that anxiety is coming into our consulting rooms. I think the last 10 couples I have assessed have all involved some kind of cyber-infidelity. We don't have the co-ordinates to know how to deal with that. What does it mean if your husband is cheating on you with a two-dimensional image on a screen? Is it the same as cheating with his secretary, as in the old days?'

Kahr has coined a term for this virtual or fantasy infidelity, the 'intra-marital' affair; he suggests it is just another thing that the increasingly embattled modern couple has to deal with. He sees fantasies about celebrities, the vicarious life of reality TV, as an extension of that impulse.

'Celebrity can have the effect of making us feel anonymous in our own life,' he says. 'The best way to get beyond that is to find a way to be a celebrity in your own household. I see a lot of couples who have spent all of their adult life together, who have slept together every night for as long as they can remember, yet they have absolutely no idea what is going on in their partner's mind. If you were filming them, it might look quite normal, but underneath, there is no intimacy. The craving for fantasy then takes root because you are afforded no celebrity within your own relationship. It's hard to keep your head together then.'

Before I go, I wonder how Kahr keeps his own head together, so full is it of other people's subterranean impulses. He suggests a couple of ways. First, he has created for himself 'a strong psychological digestive tract'; second, he never forgets the importance of 'play'. In his spare time, he writes songs for the theatre. Just before he embarked on his labour of love, he released a CD; critics compared him with Cole Porter. He would never set a patient's case to music, he says, but a faith in 'Anything Goes' romance certainly seems to help.

The statistics

Percentage of adult Britons (sample size 19,000) who have fantasised about:

· Sex with regular partner: 58%

· Sex with someone of same sex: 25%

· Sex with someone else's partner: 41%

· Sex with a friend: 32%

· Sex with a friend's partner: 23%

· Sex with a work colleague: 39%

· Sex with a stranger: 37%

· Sex with a man and a woman at the same time: 15%

· Sex with two or more men: 18%

· Sex with two or more women: 35%

· An orgy: 20%

· Being watched during sex: 19%

· Stripping off in public: 5%

· Playing a dominant or aggressive role during sex: 29%

· Playing a submissive or passive role during sex: 33%

· Tying someone up: 23%

· Being tied up: 25%

· Sex with a sister: 1%

· Sex with a brother: 1%

· Sex with animals: 3%

· Romantic scenes: 37%

· Sex & the Psyche is published on 22 February by Allen Lane, £25. To order for £23 with free UK p&p, go to observer.co.uk/bookshop or call 0870 836 0885.

· Brett Kahr will discuss his book at the ICA, London SW1 on 5 March ('Who's Been Sleeping in Your Head?,' 8.30pm. Tickets: 020 7930 3647). We have three pairs of tickets to give away. Email review@observer.co.uk, with the words 'Sex and the Psyche' in the subject field.