When the British Board of Film Classification ordered 6 minutes and 12 seconds of material cut from British Cum Queens in 2002, they found themselves under attack from an unlikely quarter: a group of feminists.

The offending segment showed some of the female participants apparently ejaculating fluid from their genitals on orgasm. The film board stated that female ejaculation did not exist, so the actresses must have been urinating. And urinating on another actor on film is banned under the UK's Obscene Publications Act.

The group Feminists Against Censorship marshalled all the scientific evidence they could find to prove that some women do in fact ejaculate. The film board eventually backed down from its complete denial of the phenomenon, stating that female ejaculation was a "controversial and much debated area".

It was only a partial climbdown, however, as the film board insisted that the scenes in question were "nothing other than straightforward urination masquerading as ejaculation". In their defence, most pornography scenes that depict women ejaculating are indeed staged. Either the fluid is put into the vagina beforehand off-camera, or the actresses are simply urinating.

The dispute raises an intriguing question. In the 21st century, when human biology has been investigated right down to the genetic level, how can the existence of female ejaculation still be open to debate?

Medical textbooks are silent on this aspect of female physiology and most physicians never learn anything of it, unless of course they experience it themselves or witness it in their partners.

In the past few years, however, there has been an upsurge of research into the female sexual response. It seems that, even today, the human body may be harbouring a few surprises.

Although still controversial, many scientists now accept that some women can ejaculate some kind of fluid during sexual arousal or orgasm. Just how common it is, what the fluid is, and whether it serves any kind of function are some of the most hotly debated questions of sex research today, and I am playing a small part in helping to investigate them.

Many historical texts, such as the Kama Sutra, spoke about female "semen", as did writers, including the Greek physician Hippocrates. Sometimes the writers may have been referring to everyday vaginal secretions, which increase during sexual arousal. However, there are several references to something more akin to ejaculation. In the 17th century, the Dutch physician and anatomist Regnier De Graaf spoke of "liquid as usually comes from the pudenda in one gush".

In the last century, Ernst Gräfenberg, the German doctor who gave his name to the controversial G spot, drew attention to female ejaculation in a 1950 paper published in The International Journal of Sexology. "This convulsory expulsion of fluids occurs always at the acme of orgasm and simultaneously with it," he wrote. "Occasionally the production of fluids is so profuse that a large towel has to be spread under the woman to prevent the bed sheets getting soiled."

Most people did not take the paper seriously and thought Gräfenberg was probably describing a type of incontinence. It is certainly true that a few women experience loss of bladder control during sex, sometimes at the moment of penetration or at orgasm. But some who end up being investigated and even surgically treated for such "coital incontinence" may in fact be experiencing ejaculation. (And probably some who think they ejaculate may in fact be leaking urine.)

It is unknown how common genuine female ejaculation might be, or even whether it occurs solely on orgasm or merely during heightened sexual arousal. Just as with men's semen, women who believe that they are ejaculating report great variation in the nature and volume of the fluid produced. It can range from clear to milky-white in colour, and the amount of fluid can range from a few drops to more than a quarter of a cup.

The real ground-breaker came in 1981, when renowned US sexologists Beverly Whipple and John Perry published a case report of a woman apparently happy to ejaculate under laboratory conditions. Watched by a team of researchers, the woman was vaginally stimulated by her husband until she reached orgasm, climaxed, and then ejaculated, releasing noticeable amounts of fluid.

According to Whipple, when Philadelphia gynaecologist Martin Weisberg saw their report he said: "Bull... I spend half my waking hours examining, cutting apart, putting together, removing or rearranging female reproductive organs... Women don't ejaculate."

In response, Whipple offered to set Weisberg up with a personal demonstration. The following is Weisberg's description of what he witnessed, which was later included in Whipple and her co-author's bestselling book, The G Spot and Other Recent Discoveries About Human Sexuality: "The subject seemed to perform a Valsalva manoeuvre [bearing down as if starting to defecate] and seconds later several cc's of milky fluid shot out of the urethra."

Impressive as that demonstration sounds, it is interesting to note that the fluid appeared to emerge from the urethra, the tube that drains urine from the bladder to an exit near the entrance of the vagina (see diagram). Could it have been urine after all?

Not according to chemical analysis of the fluid, carried out by Whipple and a few others since then. They found the ejaculate contained very low levels of urea and creatinine, the two main chemical hallmarks of urine.

One marker it did contain, however, was prostate-specific antigen, or PSA. That's the same chemical produced by the prostate gland in men.

The male prostate is usually around the size of a walnut, weighing about 23 grams. It surrounds the urethra like a doughnut and is encased by a fibromuscular sheet, which contracts during ejaculation to help expel prostatic fluid into the urethra, where it mixes with the other components of semen.

Less widely known is that women have prostate tissue too. And this, it seems, is the best candidate for the source of female ejaculate. Also known as the Skene's glands or the paraurethral glands, in 2001 the Federative Committee on Anatomical Terminology officially renamed these structures the "female prostate".

The female prostate seems to vary in size and shape much more than the male version, with some women lacking any appreciable amount of prostate tissue, according to autopsy studies by Slovakian pathologist Milan Zaviacic. This may explain women's differing experiences.

If the tissue is there at all it lies next to, or sometimes surrounds, the urethra, which is adjacent to the vagina's anterior wall. In other words, if the woman is lying on her back, the prostate is directly above the uppermost wall of her vagina.

This is roughly the same area as the G spot, the part of the vagina that is particularly sensitive to stimulation, although even the G spot's existence is controversial. Assuming there is such a thing, however, it is beginning to look to many sexologists as if the G spot is just the name for the best place to stimulate a woman's prostate. Variation in the amount of prostate tissue could explain why not all women find stimulation of this area arousing - in other words, whether or not they have a G spot