Attorney General Suella Braverman says she will continue in her role despite calling for the Prime Minister to quit.
The Cabinet minister, who has previously been a staunch supporter of Boris Johnson, told Peston on ITV that he had handled matters “appallingly” in recent days.
She said: “The balance has tipped now in favour of saying that the Prime Minister – it pains me to say it – but it’s time to go.”
She said she will put her name into the ring if there is a leadership contest.
She told Peston: “My first duty is to the country, Robert, and as attorney I’m the senior law officer.
“And we’re in a crisis and I have statutory legal and constitutional duties…
“I don’t want to resign because I have that duty. We need an attorney in government.”
Asked whether she recognises that Mr Johnson will likely sack her, she said: “That is his choice, and I will do whatever the Prime Minister asks me to do.”
The PM rejected calls to quit on Wednesday and dramatically sacked Cabinet rival Michael Gove, but was later hit with the departure of a third Cabinet minister – Welsh Secretary Simon Hart – as well as Ms Braverman’s demand.
The withdrawal of the attorney general’s support marks a significant shift by the QC, who was elected as MP for Fareham in May 2015 before being appointed as the top legal official by Mr Johnson in February 2020.
She became the first Cabinet-level minister to take maternity leave and was reappointed to her ministerial position in September.
Special legislation had to be passed by Parliament to enable her to take time off from her ministerial duties.
During her absence she was designated Minister on Leave (Attorney General) while her deputy, Solicitor General Michael Ellis, was made attorney general.
During last month’s confidence vote, Ms Braverman expressed hope the PM would win the poll with a large margin.
The Euro-sceptic had been a supporter of Mr Johnson since her days as the chair of the Brexit-backing European Research Group.
But Ms Braverman on Wednesday joined Home Secretary Priti Patel, Business Secretary Kwasi Kwarteng, Transport Secretary Grant Shapps, Northern Ireland Secretary Brandon Lewis and Welsh Secretary Simon Hart among Cabinet ministers telling Mr Johnson to stand down.