And, as a defiant Mr Sunak refused to apologise to Brianna’s family over the comments, Ms Mordaunt dramatically intervened in the row.
Her provocative words were at odds with a bid by other ministers to rally around Mr Sunak as the furore around the remarks refused to die down.
On another day of fallout over Mr Sunak’s “how to define a woman” comment:
The prime minister invited Brianna’s mother Esther Ghey to Downing Street to discuss online safety
He again refused to apologise for the joke, saying it was "sad and wrong" that the Labour leader had linked his comments to Brianna
A minister appeared to suggest Brianna’s father should “actually listen” to what the PM had said
Ms Ghey broke her silence, stressing that her “focus is on creating a positive change and a lasting legacy for Brianna”
Gordon Brown weighed in, saying he had acted when he once referred to a voter as a bigot, adding: “I made mistakes… but I apologised and I think that’s a lesson that all prime ministers should follow”
The row erupted on Wednesday when Mr Sunak accused Sir Keir in the Commons of being incapable of “defining a woman” and said it was one of many issues on which he had changed his mind.
An angry Labour leader immediately admonished the Tory leader. He said: “Of all the weeks to say that, when Brianna’s mother is in this chamber. Shame.”
Tory former minister Dehenna Davison said it was “disappointing to hear jokes being made at the trans community’s expense”, while Labour’s Jess Phillips called the prime minister “an absolute disgrace”.
Brianna’s father, Peter Spooner, demanded an apology, saying he was disgusted with Mr Sunak, before branding the comments “degrading” and “absolutely dehumanising”.
But Mr Sunak doubled down on Thursday, telling reporters: “If you look at what I said, I was very clear, talking about Keir Starmer’s proven track record of U-turns on major policies because he doesn’t have a plan.”
Mr Sunak said Sir Keir’s plan to drop a flagship commitment to spend £28bn a year on green energy demonstrated the point he was making.
Also rushing to defend the PM, policing minister Chris Philp said he had made “no reference at all to any individual trans people”.
Mr Philp accused Sir Keir of making the link to Brianna and said “anyone who is interested in this issue should actually listen to the clip; listen to the initial exchange”.
Business secretary Kemi Badenoch, who also oversees the government’s equality policy, went further, accusing the Labour leader of “trivialising” Brianna’s murder “by political point-scoring”.
And she backed the PM’s comments, saying: “It was shameful of Starmer to link his own inability to be clear on the matter of sex and gender directly to her grief.”
But in a break from her colleagues, Ms Mordaunt told the House of Commons: “Whatever the rough and tumble of this place, whatever the pressures and the stakes that are made in the heat of political combat, we owe it to the people who sent us here to strive every day to make them proud of us and this place.
“The prime minister is a good and caring man. I’m sure that he has reflected on things… that is not just about Mr and Mrs Ghey that he should reflect on, but I’m sure he is also reflecting about people who are trans or who have trans loved ones and family, some of whom sit on these green benches.”
Sir Keir insisted Mr Sunak should apologise and denied using the gaffe to score political points: “The idea that I’m responsible for Rishi’s comments at PMQs is through the looking glass. I mentioned Brianna at the very outset, because I wanted everyone to know she, Esther, was there.
“I didn’t think the prime minister would go anywhere near a comment like that knowing she was there. I really didn’t.”
Responding on her Peace & Mind UK Facebook page, Ms Ghey said her focus was solely on “creating a positive change and a lasting legacy for Brianna”.
“Through Peace & Mind, we want to improve lives by empowering people, giving them the tools they need to build mental resilience, empathy, and self-compassion through mindfulness,” she added.
“In developing these skills, I hope that we can create a more understanding, peaceful, and stronger society for everyone.”
When asked if Ms Ghey would meet with the prime minister, her spokesperson told The Independent: “Nothing has yet been set, but Esther would be absolutely open to meeting all relevant senior government officials to help drive changes in mindfulness in schools and mobile phone safeguarding for children.”
Brianna was stabbed to death by teenagers Scarlett Jenkinson and Eddie Ratcliffe in a Cheshire park last February.
During their sentencing earlier this month, the judge said the “exceptionally brutal” murder had elements of both sadism on the part of Jenkinson and transphobic hate on the part of Ratcliffe.