Rishi Sunak delivered a scathing verdict on a Tory campaign to keep Boris Johnson in Downing Street - after he was confronted by an angry party member accusing him of disloyalty.
The former Chancellor, whose departure marked the start of a mass walkout which ultimately brought down Mr Johnson, said the PM had lost the support of his own MPs.
More than 14,000 Tories have backed a campaign for the outgoing leader to be included on the ballot in the leadership contest.
Meanwhile allies of Mr Johnson believe he is already plotting a route back to power.
Mr Sunak has come under fire during the Tory fallout over the manner of the PM's departure, with Defence Secretary Ben Wallace this morning taking a swipe at ministers who walked out.
At a hustings hosted by LBC last night, an audience member reminded Mr Sunak: "He's the man that made you a senior politician" and accused him of being on "another planet".
But Mr Sunak was uncompromising - despite applause for the PM from the audience - stating: "I'd say to them that I think close to 60 people resigned in Parliament and it's incumbent on the Prime Minister to have the confidence of the parliamentary party, and that wasn't there at the end.
"So whether he's on the ballot or not, ultimately you need to be able to command the confidence of your MPs in Parliament, and we got to a point where close to 60 of them had resigned from Government."
The former Chancellor said he disagreed with Mr Johnson on economic policy, adding: "I was left with no choice but to resign."
He reiterated that he would not give Mr Johnson a place in his cabinet.
Mr Johnson remains a divisive figure as blue-on-blue infighting over the manner of his departure continues.
Liz Truss last night said that she supported the PM and professed her loyalty, while Defence Secretary Ben Wallace came out swinging for MPs who walked out.
Mr Wallace, who announced he was backing Liz Truss, told Sky News: "Some ministers don't have the luxury of walking out."
He added: "I just don't think triggering Cabinet ministers walking out at a time of a crisis is the right course of action. "There were other mechanisms to do what they wanted.
"If Rishi Sunak didn't want the Prime Minister to be Prime Minister, there are other mechanisms to do that. And that goes for all the other ministers."
Mr Johnson himself levelled a dig at Mr Sunak yesterday over his pledge to slash VAT from energy bills - despite opposing the move previously.
The PM told an audience in Birmingham: "I'll give you this assurance, they will continue with the same programme, cutting taxes, simplifying regulation as much as possible, taking advantage of all our new regulatory freedoms, getting rid of every encumbrance from solvency to MiFID to VAT on fuel - turns out to be easier than we thought."
Lord Cruddas of Shoreditch, a former Conservative Party treasurer told the Telegraph earlier this week that Mr Johnson "wants to fight the next general election as leader of the Conservative party".
He claimed the comments were made to him over lunch at Chequers last Friday.
The peer has organised a petition, calling for Mr Johnson to carry on as Prime Minister.
More than 14,000 Tory members have signed it, demanding his name be added to the ballot when members vote for a new leader next month.
A Number 10 spokeswoman said: “The Prime Minister has resigned as party leader and set out his intention to stand down as PM when the new leader is in place.”
