Britain's biggest professional body for psychotherapists has instructed members that it is unethical for them to attempt to "convert" gay people to being heterosexual, formalising a policy change long demanded by rights groups.

The British Association for Counselling and Psychotherapy has written to its near-30,000 members to inform them of the new guidelines. The letter says the BACP "opposes any psychological treatment such as 'reparative' or 'conversion' therapy which is based upon the assumption that homosexuality is a mental disorder, or based on the premise that the client/patient should change his/her sexuality". The body adds that it recognises World Health Organisation policy that says such therapies can cause severe harm to an individual's mental and physical health.

The statement, drawn up by the board of governors, ends: "BACP believes that socially inclusive, non-judgmental attitudes to people who identify across the diverse range of human sexualities will have positive consequences for those individuals, as well as for the wider society in which they live. There is no scientific, rational or ethical reason to treat people who identify within a range of human sexualities any differently from those who identify solely as heterosexual."

Conversion therapies are mainly associated with evangelical Christian groups in the US. It was long presumed that the vast majority of UK counsellors and psychotherapists recognised that these were widely discredited. But a 2009 survey of 1,300 therapists, psychoanalysts and psychiatrists found more than 200 had attempted to change at least one patient's sexual orientation, with 55 saying they were still offering such a therapy.

The BACP guideline change followed a case in which a devoutly Christian psychotherapist, Lesley Pilkington, was struck off the members' list for offering conversion therapy to an undercover journalist. Her appeal was turned down in May.

Philip Hodson, a psychotherapist and BACP spokesman, said the organisation had previously presumed its more general guidelines on equality and acting in a client's best interest would preclude members offering such therapy, but tightened up the rules after the Pilkington case.

He said: "To me, as a therapist, it seemed inconceivable that someone who had been trained and made accountable could act in that way. I was shocked rigid that a member was practising conversion therapy, which I thought only happened in wackier parts of America."

The BACP could have acted sooner, Hodson added: "There is an argument to say that we haven't looked at this when we might have done. Every single year we look at our basic and key messages, and I'm afraid this one did not rise to the top at this stage. We thought it was covered by other things."

While Pilkington's actions emerged in early 2010 the guidelines could not be changed until her appeal was completed, Hodson said: "You can't establish a formal position once a case is going on – you have to wait until a case is over."

The other main professional body for British psychotherapists, the UK Council for Psychotherapy, issued similar guidance to members in early 2010, shortly after the Pilkington case emerged.

The picture is complicated further by the fact that while the BACP and UKCP have strict accreditation and ethical guidelines, under law anyone can call themselves a psychotherapist or a counsellor. In 2007 the Labour government announced plans for statutory regulation of the professions, which would prevent this. The idea has since been dropped by the Department for Health.

On Monday the governor of California, Jerry Brown, signed off a law banning any "conversion" therapies on those aged under 18.