A woman who alleges that she was inappropriately prescribed sex-changing hormones and then wrongly underwent a double mastectomy is one of several complaints being investigated by the General Medical Council about the doctor who oversaw her aborted gender reassignment, the Guardian has learned.

The GMC, the doctors' professional regulator, has received at least three separate complaints against Dr Richard Curtis, a London GP who specialises in the treatment of gender dysphoria, particularly transsexualism. The complaints concern the alleged inappropriate administering of sex-changing hormones to patients and at least one allegedly unsuitable referral for gender reassignment surgery.

It is claimed that Curtis, who provides private treatment to patients seeking gender reassignment, failed to follow accepted standards of care and breached conditions placed on his practice by the Medical Practitioners Tribunal Service (MPTS), the GMC's arm's-length disciplinary body.

The allegations include commencing hormone treatment in complex cases without referring the patient for a second opinion or before they had undergone counselling, administering hormone treatment at patients' first appointments, and referring patients for surgery before they had lived in their desired gender role for a year, as international guidelines recommend. One patient allegedly underwent surgery within 12 months of their first appointment. He is also accused of administering hormones to patients aged under 18 without an adequate assessment, and wrongly stating that a patient seeking gender reassignment had changed their name.

One of the most serious cases concerns a female patient who regrets switching to a male role. She underwent hormone treatment and had her breasts removed. The woman is one of the complainants in the current GMC investigation.

Other cases include patients who were allegedly prescribed hormones at age 16. Although the Tavistock and Portman NHS Foundation Trust in London, which specialises in the treatment of gender identity difficulties in children, does offer hormone treatment to under-18s on the NHS, it is alleged that Curtis lacks the specialist knowledge and skills to adequately treat such patients on his own.

The MPTS first imposed conditions on Curtis in November 2011, restricting his practice in the treatment of patients seeking to switch their gender role. An interim orders panel then decided he "must maintain an anonymised log detailing every case where he prescribes for patients with gender dysphoria and for patients who he refers for gender dysphoria surgery".

The panel also ruled: "He must not prescribe hormonal treatment for patients with gender dysphoria, or refer any patients for gender dysphoria surgery, unless those patients have undergone a recent mental health or psychological assessment carried out by an appropriately trained mental health care professional."

In January 2012 another interim orders hearing committee extended the conditions placed on Curtis until 3 May 2013. The restrictions were upheld by another hearing on 7 December and will remain in place pending the outcome of a GMC investigation. If the council decides there is a case to answer, it will refer the investigation back to the MPTS, which will then convene a fitness-to-practice hearing.

A spokeswoman for the Medical Defence Union, which is representing Curtis, said the doctor was unable to comment on any of the allegations "because of the ongoing investigation and his duty of patient confidentiality". She added: "He has no further comment to make."

Curtis qualified in 1991 from St Bartholomew's Medical College, London, and became a general practitioner in 1995. He took over the London Gender Clinic, the largest private transgender clinic in the UK, five years ago. Its website states that the clinic "provides comprehensive health care to gender dysphoric, gender variant, transvestite, transgendered and transsexual people", including "providing hormonal prescriptions and referrals for transsexual surgery, gender counselling and hair removal treatment".

The case seems likely to revive the furore over the treatment of gender dysphoria in the UK triggered by the GMC inquiry against Dr Russell Reid, a psychiatrist who provided private treatment for transsexualism. Reid was found guilty of serious professional misconduct in 2007 for breaking international guidance between 1988 and 2003.

That case divided transsexual experts and support groups. It was brought after complaints from four psychiatrists, two of whom were colleagues of Reid at the Charing Cross hospital gender identity clinic, and one of the patients.

Curtis took over Reid's private practice in Earls Court, west London, in February 2006. In a press release issued in November 2005, Curtis said: "I will be practising according to the Harry Benjamin Guidelines but will treat each case sensitively and advise according to an individual's specific circumstances much in the same way as Dr Reid."

Curtis's website states he is a member of the World Professional Association for Transgender Health (WPATH), formerly known as the Harry Benjamin International Gender Dysphoria Association. The association's standards of care are accepted in most countries as the benchmark for treatment of gender non-conformity and gender dysphoria, including transsexualism. Patients should undergo psychological assessment before taking sex-changing hormones and be properly monitored for adverse effects, which can include heart problems, blood clots and high blood pressure.

Patients seeking breast or genital surgery should have "persistent, well-documented gender dysphoria", be over the age of consent, and have the mental capacity to consent to the treatment. They should live full-time in their desired gender role for at least a year before surgery, to see how they cope with work, family, friends and relationships. But the guidelines are not legally binding but flexible directions which can be modified to suit a patient's needs.

In 2002 a survey revealed wide variations in the treatment of transsexualism across Britain, with some clinics offering "surgery on demand" and others no surgery. Subsequently an intercollegiate working committee, bringing together experts from a range of royal colleges and professional bodies, including the Royal College of Psychiatrists, was convened to draw up UK specific guidelines to address the concerns about the quality and availability of care.

Following almost a decade of consultation among health professionals and with the transgender community, national standards have now been agreed, said the committee chairman, Professor Kevan Wylie, a consultant psychiatrist in Sheffield. He said: "These are now going through the final stages of endorsement from the various colleges, societies and stakeholders that have been involved. I would envisage them being released early next year."