Harry Potter author J.K. Rowling angered the trans community on Monday for comparing hormone treatment for people seeking to transition gender to gay conversion therapy, fueling a row over transgender rights.

"We are watching a new kind of conversion therapy for young gay people," the author tweeted on Sunday, referring to hormone prescriptions for youths questioning their gender identity as the "new anti-depressants".

Her comment sparked a furious reaction from trans rights campaigners, who have criticised Rowling's remarks on transgender issues repeatedly in recent weeks.

"The suggestion that anyone would subject a child to gender-changing treatment to avoid them growing up gay or lesbian is a slander on all the caring parents," said Christine Burns, an advocate who has campaigned for trans rights since the 1990s.

"The notion is so far down the fantasy rabbit hole that it's impossible to address in a sentence or two.

"It's a lie that relies on a mountain of lies. The scandal is that these lies go unchallenged," Burns told the Thomson Reuters Foundation.

Rowling's public relations manager said on Monday that she would not be commenting further.

Rowling, whose Harry Potter series has sold more than 500 million books, waded into the debate over whether trans rights encroach on those of other women with a June tweet over a headline about "people who menstruate".

"'People who menstruate.' I'm sure there used to be a word for those people. Someone help me out. Wumben? Wimpund? Woomud?" Rowling tweeted.

The author, who has also penned the popular Cormoran Strike detective series under the pseudonym Robert Galbraith, followed the tweet with a 3,600-word essay, in which she disclosed she had been a victim of domestic abuse and sexual assault.

In response to Rowling's latest comments, model and trans activist Munroe Bergdorf said the writer was "dangerous" and a "threat to LGBT people".

"J.K. Rowling is not a scientist. She is not a doctor. She is not an expert on gender. She is not a supporter of our community," Bergdorf said on Twitter.

"She is a billionaire, cisgender, heterosexual, white woman who has decided that she knows what is best for us and our bodies. This is not her fight."

Scott Leibowitz, child and adolescent psychiatrist at the Nationwide Children's Hospital in Columbus Ohio, said Rowling's comments sent a worrying message to trans youth, many of whom would have come into contact with the author through her books.

"For a trans young person wanting acceptance from family members, a statement like this can give skeptical parents a reason to reject this aspect of them, thinking they will 'grow out of it', because a famous person said so," Leibowitz said.