Sir Keir Starmer has come in for criticism after saying it is "not right" to insist that only women have a cervix.
The Labour leader made the controversial comments while speaking to Andrew Marr on Sunday morning.
He was asked on The Andrew Marr Show on BBC 1 whether it was transphobic to say that only women have a cervix.
Starmer replied: "It is something that shouldn't be said. It's not right."
He spoke to the BBC as the Labour Party holds its annual conference in Brighton.
Controversy surrounded the event even before it began when Rosie Duffield, Labour MP for Canterbury, announced that she was withdrawing over threats from trans activists.
Last year, she tweeted that "only women have a cervix".
In further comments to the BBC, Starmer said he had spoken to Duffield ahead of the conference and told her it was a "safe place for her to come".
But he said there needed to be "a mature, respectful debate about trans rights" and that transgender people were among the most "marginalised and abused" in society.
He said, "We do everybody a disservice when we reduce what is a really important issue to these exchanges on particular things that are said."
Health secretary Sajid Javid has waded into the row, saying on Twitter that the Labour leader's comments are a "total denial of scientific fact".
His response was praised by Maya Forstater, a woman who lost her job over trans-critical tweets.
"Thank you for saying so. [It] is unbelievable that in 2021 it is considered controversial that only women have a cervix," she said.
Angela Richardson, Tory MP for Guildford, replied "Bravo".
Professor Patrick Sturgis, Professor of Quantitative Social Science at the London School of Economics, said, "This simply is not a sustainable position."
Total denial of scientific fact.— Sajid Javid (@sajidjavid) September 26, 2021
And he wants to run the NHS. https://t.co/zdQjJU55r3
The question to Starmer came after Duffield called on him to clarify his views on transgender rights.
"I think many Labour women wanted a clear commitment and still haven't heard that," she said, according to The Times.
"It was disappointing that the discussion was framed solely around trans rights rather than women's rights, which is what concerns me most about this issue.
"Women now need to hear clearly from Keir Starmer that the Labour Party will listen and respond to our concerns about upholding and protecting single-sex services as outlined in the 2010 Equality Act."
More on this topic: If politicians can't stand up for Rosie Duffield, surely the Church should
The Archbishop of Canterbury, Justin Welby, has called for people to "disagree well".
"Absolutely everybody has the right to be safe from abuse, threat or harm. That includes Rosie Duffield AND the transgender community," he said on Twitter.
"It's about time we looked for our shared humanity in our dealings with others, rather than the division."
Earlier this year, Starmer attracted criticism after apologising for a visit to Jesus House church in London because it has a traditional view of marriage.
He also deleted a video from his Twitter account that showed him visiting a pop-up vaccination clinic at the church.
The retraction was made after he came under pressure from LGBT Labour.
Rev Yemi Adedeji, a Trustee of London School of Theology, said at the time that it was "absolutely disgraceful" that Starmer had been "placed under pressure to reject both the church and the African community for their heterosexual belief".