James Bellringer never planned to cut up penises for a living. Since 2000, the surgeon has performed 200 male-to-female sex change operations. "I'm surprised there are any men left!" he jokes.

Around 5,000 people in Britain have had a sex change, formally known as gender reassignment surgery, of whom about 4,500 are male to female (MTF) transsexuals. Between 300 and 400 operations are performed annually, around half on the NHS. Demand for the operation is rising. The number of referrals for surgery at the UK's main gender identity clinic (GIC), where Mr Bellringer works with a team of psychiatrists, has doubled to 1,000 over the past four years. He says: "I did about 70 operations last year and will do nearer 100 this year."

MTF sex changes have been performed in Britain since the 1960s. The operation was by then considered relatively straightforward by surgeons in the USA. The surgery involves removal of the testicles (orchidectomy) and the penis (penectomy) before creating a neo-vagina with skin taken from the penis, the scrotum or, more rarely, the bowel - a process known as vaginoplasty. The tip of the penis is used to make an artificial clitoris (clitoroplasty).

After training in urology, which covers problems including impotence and incontinence, Mr Bellringer was approached by Charing Cross GIC four years ago to become their new gender reassignment surgeon. His predecessor Mike Royle provided him with on the job training before retiring.

Mr Bellringer says that only his older patients, who do not wish to be sexually active, chose not to have the full surgery and just have their male genitalia removed. He said that around a third of his patients regard themselves as heterosexual women and require a vagina adequate for penetrative sex. Many others are never sexually active but want a vagina to feel properly female. A third group, who identify as lesbian, often request a smaller vagina, he adds.

In recent months concern has grown about the benefits of gender reassignment surgery. A systematic review of medical research on sex change operations by the Univeristy of Birmingham's aggressive research intelligence facility, published last week, found no robust scientific evidence the procedure was beneficial.

Does Mr Bellringer have concerns about the quality of the research which underpins his practice? He admits there is a need for more scientific evidence, but says that hormones and surgery are the only treatments found to have helped transsexuals. The surgeon also points to ongoing research in Holland, which suggests that part of the brain in MTF transsexuals resembles that of born women. "There's evidence of a brain abnormality among transsexuals," he says. "They've done about eight or nine post-mortems on MTF transsexuals and found a nucleus in the brain which looks like those in born women."

But other medical studies from Holland and the USA suggest that up to a fifth of patients regret changing sex. Has Mr Bellringer ever come across such cases? "They're extremely rare," he says. "Only one of my patients has expressed regret about becoming a woman, but they didn't want their penis back either. He is living as the male partner in a relationship but is very happy having no genitalia.

"There's genuinely a group of patients who wish to be asexual. They're an intermediate gender. Psychiatrists are beginning to recognise this. I had another person who just wanted their testicles removed. I wasn't sure about this but he was assessed by four or five psychiatrists who decided that it was ok."

The surgeon has seen another two patients who have resumed living in the male role. He says that one of the patients was "quite old" and his neo-vagina has narrowed, partly because he is not sexually active. The other patient is a Jehovah's witness and was told by fellow believers that he could not turn up to their meetings other than as God intended, in other words as a man. Mr Bellringer says that neither patient is "desperately unhappy" and quite comfortable with their lives.

Guidelines set by the Harry Benjamin International Gender Dysphoria Association, based in Minneapolis, set minimum criteria for gender reassignment to prevent inappropriate sex changes. The standards of care state that patients should undergo three months of psychotherapy before taking sex-changing hormones. They should then live full-time in their desired gender role for at least a year before surgery, a period known as the real life experience (RLE), to see how they cope with work, family, friends and relationships.

Patients at Charing Cross, and other NHS gender clinics, face more stringent criteria, including a two-year RLE. Mr Bellringer believes this helps to prevent patients having surgery inappropriately. He says: "Many patients make it through the first year but there's a high drop out rate during the second year."

However, the surgeon says that the vast majority of his patients go on to lead much happier lives following surgery. "The number who come back and express immense amounts of gratitude is overwhelming. I regularly get thank you cards and three or four Christmas cards. It makes my work really rewarding."