Boris Johnson has said he thinks there is a difference between women and transgender women.
In comments to reporters en route to Rwanda, the Prime Minister also backed the ban on transgender athletes competing in women's swimming events if they have gone through male puberty.
Asked whether he thought a woman can be born with a penis, Mr Johnson said, "Not without being a man, that's my view about that."
When reporters asked if he thought there was a difference between a trans woman and a biological woman, he said, "Yes."
The Prime Minister said it was important to be "understanding" towards transgender people but that there were "particular problems" around gender identity.
He added that these require great sensitivity.
"Look it's very, very important that as a society we should be as understanding of everybody else as possible. I've always stood for that," he said.
"When you start to move from issues of sexuality to issues of gender you start to raise particular problems.
"I think I've spoken of three concerns I've had in the past. They are to do with the age at which you can (become) Gillick competent to transition, the question of safe spaces for women, and the difficulties you have in sporting competitions.
"These are all very difficult problems and you have to be very, very sensitive."