Sir Keir Starmer said some women have a penis at the weekend, in an interview set to cause friction in the Labour Party.
The Holborn and St Pancras MP has repeatedly found himself struggling to define what a woman is since taking over the role of Opposition Leader in 2020.
On several occasions he has sought to swerve the question altogether as he aimed to avoid upsetting the transgender community or women concerned about the deterioration of their rights.
But at the weekend Starmer finally gave a clear answer, siding with trans activists.
Sir Keir Starmer said his view was not about rolling back women's rights
"For 99.9 per cent of women, it is completely biological... and of course they haven’t got a penis," he told the Sunday Times.
"Some people identify as a different gender to the one they are born with."
Vowing to protect women's rights, he added: "I think there is a fear that somehow there could be the rolling back of some of the things that have been won.
"There are still many battles that need to go ahead for women and I don't think we should roll anything back."
He continued: "But simply turning it into a toxic divide advances the cause of no one - the cause of women or those that don't identify with the gender that they were born into."
His comments come just weeks after former Scottish First Minister Nicola Sturgeon saw her popularity drop after introducing legislation that would have allowed Britons north of the border to self-identify their gender.
Under the laws those over the age of 16 would be able to legally apply for a Gender Recognition Certificate without the sign off of a medical professional.
Introduction of the legislation was blocked by Westminster.
Labour MP Rosie Duffield has been critical of the party's stance on trans issues
Starmer has himself previously committed to introducing "self-declaration" for trans people.
Labour advisers are thought to have since privately warned their party leader that such a policy risks costing them the general election.
The party's Canterbury MP, Rosie Duffield, even went public to accuse Labour of having a "woman problem" for the leadership's handling of trans issues.
"Keir may dismiss this as a culture war issue, but for these women, it is most definitely not,” she said earlier this year.
"Many of us know that self-identifying as a woman does not make a person a biological woman who shares our lived experience."