More than 370 religious leaders from around the world are calling for a ban on conversion therapy – the attempt to change a person’s sexual orientation or gender identity.
The signatories to the declaration represent all the world’s major faiths and many are known LGBT advocates.
They include South African cleric Archbishop Desmond Tutu and former Chief Rabbi of Ireland David Rosen.
Other religious figures said a ban could risk criminalising pastors.
A declaration calling for a ban will be launched at a conference sponsored by the Foreign, Commonwealth and Development Office (FCDO) on Wednesday.
The Anglican Bishop of Liverpool, Paul Bayes, and Mary McAleese, the former president of Ireland, are also among those who have signed the declaration.
Prime Minister Boris Johnson has repeated a promise to ban conversion therapy, saying in July that the practice was “absolutely abhorrent” and “has no place in this country”.
The government has yet to publish details of the ban but said it had commissioned research and would outline its plans “in due course”.
The term “conversion therapy” refers to any form of treatment or psychotherapy which aims to change a person’s sexual orientation or to suppress a person’s gender identity.
It can range from electric shock treatment to religious teachings or talking therapies designed to change someone’s sexuality.
The practice is already outlawed in Switzerland and areas of Australia, Canada and the US.