Ex-Tory Cabinet minister Jacob Rees-Mogg has accused Rishi Sunak of behaving like the Borgias - a notorious Italian family known for treachery.
The remark came as the Prime Minister faced anger from Tory MPs for a humiliating U-turn on a leadership pledge to scrap thousands of EU-era laws by the end of 2023.
Business Secretary Kemi Badenoch announced last night around 600 laws would be revoked - rather than the 4,000 pledged - insisting it is "about more than a race to a deadline".
Speaking to BBC Radio 4's Today programme, Mr Rees-Mogg, who was in charge of the plans during Liz Truss's Government, said: "Politicians need to stick to what they said they will do.
"When Rishi Sunak resigned [as Chancellor], he said in his resignation letter to Boris Johnson that he believed the public are ready to hear the truth - our people know if something is too good to be true, then it's not true."
He added: "He then said something that people like me wanted to hear, and has failed to deliver it. I'm afraid it's no good being holier-than-thou if you end up behaving like a Borgia."
The House of Borgia, which produced two Popes, was a wealthy aristocratic family during the Italian Renaissance and some members became known for their treachery.
Asked whether the Conservative Party is doomed under Rishi Sunak's leadership, Mr Rees-Mogg added: "I want him to succeed, I wanted him to do well in the local elections.
"This is why I kept quiet on retained EU law until after the local elections were over."
During an urgent question on Thursday, the Tory MP Mark Francois asked the Business Secretary: "What on earth are you playing at?".
Ms Badenoch was also rebuked by the Speaker Lindsay Hoyle for unveiling the U-turn through a written statement - rather than to MPs.
After apologising to the Speaker for it not being "to your satisfaction", he shouted back at her: "Who do you think you're speaking to?".
But the climbdown was cautiously welcomed by critics in business and environmental groups who had warned the project was unfeasible and that important regulations would be scrapped without proper scrutiny.
Attempting to defend Mr Sunak the Home Office minister Sarah Dines also claimed on Thursday the Government's move is "not quite a U-turn".
She told GB News: "We need to make sure we get rid of those laws which we don't need and we keep some which are compatible with what we want to do in this country."
