Campaigners and medical professionals have welcomed California Governor Jerry Brown’s decision to ban teenagers from accessing discredited treatments that seek to reject an LGBT identity.

When the law comes into effect on 1 January 2013, California will become the first to outlaw the practice for people under the age of 18 in America.

The bill was signed over the weekend and has the support of mental health groups and gay rights activists.

In a statement, Mr Brown said gay conversion therapy had “no basis in science or medicine,” and that it would be “relegated to the dustbin of quackery”.

The Human Rights Campaign said: “We’re grateful to Governor Brown for standing with California’s children.

“LGBT youth will now be protected from a practice that has not only been debunked as junk science, but has been proven to have drastically negative effects on their well-being”.

However, the National Association for Research and Therapy on Homosexuality said the bill was a “legislative over-reach”.

In July 2011, Christian therapist Lesley Pilkington was found guilty of malpractice by the British Association of Counselling and Psychotherapy (BACP), after she was shown in 2010 to be providing gay conversion treatments following an undercover investigation by the journalist Patrick Strudwick.

Mrs Pilkington told Mr Strudwick that homosexuality was a “mental illness” and she could help him overcome it.

In May of this year, Mrs Pilkington lost an appeal against the BACP.

The medical body had described her as “negligent”, “dogmatic” and “unprofessional” in her behaviour.

Lesley Pilkington was backed in her appeal by former Archbishop of Canterbury Lord Carey, who helped launch the Coalition for Marriage campaign to oppose equal marriage rights for gay couples.

Lord Carey signed an open letter which said: “Competent practitioners, including those working with biblical Judeo-Christian values, should be free to assist those seeking help.”