The Californian governor has signed off on new laws making it illegal to use psychotherapy in an effort to make gay teenagers straight.

California is the first state in the US to pass a law banning conversion therapy on anyone under 18.

The state's governor, Jerry Brown, says therapies used to try to change a person's sexual orientation have no basis in science or medicine.

"They will now be relegated to the dustbin of quackery," Mr Brown said on his Twitter feed, after signing the ban into law at the weekend.

Gay rights activists have been battling groups that believe counselling and prayers will help teenagers rid themselves of their homosexuality, arguing the treatment can cause depression and suicide.

Under the law, doctors cannot provide therapy that seeks "to change behaviours or gender expressions or to eliminate or reduce sexual or romantic attractions or feelings toward individuals of the same sex".

The law comes into effect in January.