Plans to ban controversial "gay conversion therapy" will be brought forward after the government completes a "study" on the issue, the PM says.
Boris Johnson said the practice was "absolutely abhorrent" and "has no place in this country".
His comments come two years after the government pledged to ban it as part of its LGBT equality plan.
Campaigners have said they hope the ban on "conversion therapy" covers all LGBT people, including trans people.
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.
All major UK therapy professional bodies and the NHS disagree with it on logical, ethical and moral grounds.
Mr Johnson said: "On the gay conversion therapy thing, I think that's absolutely abhorrent and has no place in a civilised society, and has no place in this country.
"What we are going to do is a study right now on, you know, where is this actually happening, how prevalent is it, and we will then bring forward plans to ban it."
On the government's response to a consultation on the Gender Recognition Act, Mr Johnson said it will be published "over the summer".
The consultation was launched in 2018 in order to understand gender in more detail and explore ways in which legal recognition for trans people in England and Wales could be improved.
The Ban Conversion Therapy campaign said: "We're delighted that Boris Johnson has listened to our calls for a ban on conversion therapy.
"Now we await action - LGBTQ+ people have been tortured for long enough."
Gendered Intelligence, a charity that aims to improve the quality of life of trans people, said: "Whilst we hope the prime minister used 'gay conversion therapy' for brevity, we must ensure the ban on conversion therapy covers all LGBT people, including trans people.
"We have to make sure we don't allow trans conversion therapy through the backdoor."
Layla Moran, a Liberal Democrat leadership candidate who revealed in January she was pansexual and in a relationship with a woman, said the "conversion therapy" remarks were welcome but "there have been 'plans' and 'pledges' for over two years now".
"What we need is urgent government action to end this outdated and harmful practice for good," she said.
In 2018, it was announced that controversial "gay conversion therapies" were to be banned as part of a government plan to improve the lives of gay and transgender people.
At the time, the government did not offer a definition of "conversion therapy", but its report said it "can range from pseudo-psychological treatments to, in extreme cases, surgical interventions and 'corrective' rape".
A national survey of 108,000 members of the LGBT community suggested 2% have undergone the practice, with another 5% having been offered it.