The PM's words come after the government excluded transgender people from a conversion therapy ban, ITV News' Libby Weiner reports
Boris Johnson has said "biological males" should not be competing in women's sports, amid controversy over two elite transgender athletes seeking to take part in cycling and swimming events.
The prime minister accepted the debate on the rights of trans people presents "complex issues" and his opinions may put him "in conflict" with many in the LGBT+ community.
But, he said, "that doesn't mean that I'm not immensely sympathetic to people who want to change gender, to transition".
His comments come after British cyclist Emily Bridges, who was assigned male at birth before seeking to transition, was declared ineligible to race at the British National Omnium Championships.
And in America, swimmer Lia Thomas, who was also assigned male at birth, sparked debate after she won the women’s 500-yard freestyle in the NCAA swimming championship in Atlanta.
Both athletes were competing in the male categories of their sports until relatively recently.
They have both transitioned and have gone through the relevant hormonal treatment allowing them to compete, but some campaigners say they still have an unfair advantage having been assigned male at birth.
Prime Minister Johnson, asked to explain his decision not to include trans people in the government's proposed ban on conversion therapy, gave an unprompted opinion on trans athletes.
"I don't think that biological men should be competing in female sporting events," he said.
"Maybe that’s a controversial thing but it just seems to me to be sensible."
He added: "And I also happen to think that women should have spaces which are – whether it’s in hospitals or prisons or changing rooms or wherever – which are dedicated to women."
Bridges - winner of the men's points race at the British Universities' championships in Glasgow in February - began hormone therapy last year.
She said she'd been far below the testosterone limit required to compete in the British National Omnium Championships but was told two days before that she would not be allowed.
In a statement posted on Twitter by her mother, Bridges explains how she has been "relentlessly harassed and demonised by those who have a specific agenda to push."
The Welsh cyclist went on to explain how "no one should have to choose between being who they are, and participating in the sport that they love".
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She added: "I have been judged despite a total lack of evidence against me, purely because I am trans."
On conversion therapy, the PM said: "I don't think that it's reasonable for kids to be deemed so-called Gillick-competent to take decisions about their gender or irreversible treatments that they may have. I think there should be parental involvement at the very least."
He added: "It's vital that we give people the maximum possible love and support in making those decisions.
"But these are complex issues and I don't think they can be solved with one swift, easy piece of legislation. It takes a lot of thought to get this right."