Rishi Sunak has been criticised for aiming a political jibe about transgender people at Labour leader Sir Keir Starmer - while Brianna Ghey's mother Esther attended PMQs.
In Prime Minister's Questions, his weekly showdown with MPs and the leader of the opposition, Mr Sunak said the Labour leader had broken promises on "defining a woman".
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Mr Sunak said: "We are bringing the waiting lists down for the longest waiters and making progress, but it is a bit rich to hear about promises from someone who has broken every single promise he was elected on.
"I think I have counted almost 30 in the last year. Pensions, planning, peerages, public sector pay, tuition fees, childcare, second referendums, defining a woman - although in fairness that was only 99% of a U-turn."
Sir Keir has previously said that 99.9% of women "haven't got a penis".
Responding to the prime minister, Sir Keir said: "Of all the weeks to say that - when Brianna [Ghey]'s mother is in this chamber.
"Shame."
Sir Keir added: "Parading as a man of integrity when he's got absolutely no responsibility.
"I think the role of the prime minister is to ensure that every single citizen in this country feels safe and respected, it's a shame that the prime minister doesn't share that."
Mr Sunak's press secretary later denied the remark was transphobic.
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Sir Keir had already mentioned that Ms Ghey was attending PMQs in the Commons - although it is unclear if she was present in the chamber at the time the jibe was made.
The Labour leader started the session by saying: "This week the unwavering bravery of Brianna Ghey's mother Esther has touched us all. As a father, I can't even imagine the pain that she is going through and I am glad that she is with us in the gallery here today."
During the sentencing of Brianna Ghey's killers, the judge said she had taken into account the "sadistic" and "transphobic hostility" of her killers.
Mr Sunak was later asked to apologise by Labour MP Liz Twist - but did not do so.
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He ended the session in the Commons by reiterating his sympathy for Esther Ghey.
Mr Sunak said: "I could just say also to Brianna Ghey's mum, who is here, as I've said earlier this week, what happened was an unspeakable and shocking tragedy, Mr Speaker, and as I said earlier this week in the face of that, for her mother to demonstrate the compassion and empathy that did last weekend, I thought demonstrated the very best of humanity in the face of seeing the very worst of humanity.
"And she deserves all our admiration and praise for that."