Its exhaustive research has concluded that men and women have a fundamentally different relationship between arousal and desire. A women's arousal is triggered by a network of emotional, intellectual and relationship-based factors rather than the simple physical response required by a man, it says.

Dr Mitra Boolel, the leader of the company's sex research team, admitted: "The brain is the crucial sexual organ in a woman."

While a man's arousal almost always led to a desire for sex, there was no such obvious corresponding factor with women, he said.

"There's a disconnect in many women between genital changes and mental changes," he explained. "This disconnect does not exist in men. Men consistently get erections in the presence of naked women and want to have sex. With women, things depend on a myriad of factors."

Pfizer has not given up all hope of finding an alternative to Viagra for women. Dr Boolel has now instructed his research team to concentrate on finding drugs that affect a woman's brain chemistry. Dr Joe Feczko, the president of Pfizer's worldwide development, said that female arousal disorders were "far more complex than male erectile dysfunction".

He added: "Diagnosing female Sexual Arousal Disorder involves assessing physical, emotional and relationship factors, and these complex and interdependent factors make measuring a medicine's effect very difficult."

Pfizer, the world's largest private medical research and development organisation, spent hundreds of thousands of pounds on its research.

When it began its quest in 1996, the company had high hopes that Viagra, or a slightly altered version of it, would work for women as well as men. The anti-impotence drug (which sells for about £5 a pill) has been prescribed for more than 23 million men worldwide. It generates about $1 billion in sales a year for Pfizer.

In early trials, where women were dosed with Viagra while watching erotic videos, the drug appeared to work. However, in later studies the Pfizer team found that even though Viagra induced a greater pelvic blood flow, the women did not feel substantially more aroused.

Their conclusion that the earth only moves for women if they think it does came as little surprise to Kathy Lette, the author of books such as Foetal Attraction and Dead Sexy.

"Women have an emotional libido and we've known that for decades," she said. "It's just a question of when men will cotton on. To please a woman in bed, all a man has to do is a poetry course. They also have to learn that the Karma Sutra is not an Indian takeaway and that the mutual orgasm is not an insurance company."

Anne Atkins, the broadcaster and commentator on moral and ethical issues, agreed that the Pfizer findings were "blindingly obvious". "I'm surprised we needed scientists to tell us this," she said. "Women have always known that men are turned on in a very straightforward way.

"Women get satisfaction from something much deeper - from relationships involving romance and love. While men also desire that, they are not programmed for it in the same way."

Mrs Atkins added that she believed a female Viagra was possible, but that the invention of such a drug would be a "retrograde" step.

"It would be one more instance of trying to turn women into surrogate men, in much the same way as the sexual revolution made us more promiscuous," she said. "We can train ourselves to be more promiscuous and more like men in our sexual desires, but I don't think we are any the happier for it. I don't think an artificial libido enhances our lives.

"Buying us a bunch of red roses is far more likely to work and is far healthier for all of us."

The Marquess of Bath, the owner of Longleat House who enjoys a coterie of "wifelets", said that he would reserve judgment on the findings until he had conducted his own research.

"This is the first time that I've heard this [that the brain is the crucial sexual organ for women], and I don't necessarily think that it's true," he said.

Leonore Tiefer, a New York-based clinical psychologist, said that any attempt to come up with a female Viagra was doomed to fail.

"These companies believe that sex is a matter of organ function, like breathing or excretion," she said. "I don't think there's an answer to the question of what women should want sexually."

Pfizer nevertheless insisted that the search would go on for a female Viagra. "Pfizer has made a number of major contributions to the emerging science of female sexuality," Dr Feczko said. "We now have a better understanding of the biochemical and physiological factors involved in female sexual function."

Kathy Lette advised Pfizer not to bother. "A cheap and widely available tablet to restore the female sex drive already exists," she advised. "It's called paracetamol."