Legislation is being discussed in Parliament to make it illegal for registered counsellors or psychotherapists to help people with unwanted homosexual attractions.

The Counsellors and Psychotherapists Regulation Bill contains a clause stating that a code of ethics for all registered therapists "must include a prohibition on gay to straight conversion therapy".

The bill's author, the Labour MP for Swansea West Geraint Davies, has said in Parliament that the intention of the bill is to better regulate an industry that is at present dangerously unmonitored, and therefore has the potential to do a great deal of damage.

Mr Davies has said: "This year a million Britons will seek psychotherapy, but currently anyone can set themselves up as a psychotherapist with no training or recourse for the patient when something goes wrong.

"This gives a free rein to those offering bogus treatments like conversion therapy. It's a scandal that someone can offer this discredited, so-called treatment and still call themselves a professional therapist."

Margaret Unwin, of the LGBT support charity PACE has said: "We are fully behind the sensible proposal of Mr Davies to outlaw conversion therapy, which has been condemned by the British Association of Counselling and Psychotherapy and the UK Council of Psychotherapy."

She added: "All people have a right to express their sexual orientation, and we look forward to the day when people will stop being stigmatised because of who they love."

Christian Concern said the law was a "Trojan Horse" and "tyrannical" in its attempt to enforce the ideology that sexuality is fixed at birth.

Andrea Williams, the charity's chief executive, has accused Mr Davies of seeking to condemn many individuals to a life of misery because they wish to live differently.

"This is a bill without any compassion for those individuals who feel, for a variety of reasons, they do not want to live a homosexual lifestyle," she said.

"These are men and women who would be condemned to a lifestyle that makes them desperately unhappy and be prevented from finding proper help.

"Vulnerable people would be forced to seek out back-street remedies for their unwanted feelings rather than having the freedom to consult professional counsellors.

"The irony of this bill is that it claims to stop people being 'forced to change their sexual identity from homosexual to straight'. But the reality is that for very many people who do have same-sex attraction, their sexuality is not fixed, and is certainly not their total identity in life.

"This is a bullying piece of legislation which tells vulnerable people they must live a gay life – by law."

The debate about the nature of the origin of sexuality in humans is complex and ongoing. Prominent LGBT activist Peter Tatchell has in the past spoken of the lack of evidence surrounding a biologically determined sexuality.

In a Guardian article from June 2006, he said: "If genes determine our sexual orientation, we would expect that in cases of identical twins where one was gay the other would be gay too - in every case.

"But, in fact, in only just over half the cases are both twins gay. The same lack of complete concordance is found in hormone-associated physical attributes."

Mike Davidson, Director of Core Issues Trust, who has himself moved away from same-sex attractions and activity, said, "I can hardly believe that our Parliament is contemplating legislation that you might expect in a dictatorship, forcing people with feelings they don't want to keep.

"Those who do not want to live as a homosexual are not motivated by ideology but by a heart-felt desire to live differently. And I can point to many case studies where caring, professional counselling has enabled individuals to live fulfilled lives in the way they prefer to live."

The Counsellors and Psychotherapists Regulation Bill is expected to have its second reading on 24 January 2014.