Rishi Sunak is facing calls to apologise after joking about Labour's position on trans people when the mum of murdered teenager Brianna Ghey - who was transgender - was in Parliament.
During Prime Minister's Questions, Mr Sunak ridiculed Sir Keir Starmer for U-turning on "defining a woman".
No 10 said it was "legitimate" to question the Labour leader's position.
But Brianna's father, who is among those calling for an apology, said the PM's comment was "degrading".
Her father, Peter Spooner,
told Sky News: "As the prime minister for our country to come out with degrading comments like he did, regardless of them being in relation to discussions in Parliament, they are absolutely dehumanising.
"Identities of people should not be used in that manner, and I personally feel shocked by his comments and feel he should apologise for his remarks."
Brianna's family are being invited to a meeting about online safety with the prime minister and Technology Secretary Michelle Donelan, the BBC has been told.
At PMQs, Mr Sunak listed what he described as a number of Labour U-turns, accusing Sir Keir of changing his position on "defining a woman".
"Although in fairness, that was only 99% of a U-turn," he added.
Sir Keir hit back, saying: "Of all the weeks to say that, when Brianna's mother is in this chamber. Shame.
"Parading as a man of integrity when he's got absolutely no responsibility."
Brianna's mother Esther was not in the public gallery for the exchange but entered shortly afterwards.
Later, Mr Sunak ignored a call from Labour MP Liz Twist to "apologise to Brianna Ghey's mother".