Simon Clarke has called on Rishi Sunak to be ousted as prime minister.
The former cabinet minister said the Tories “will be massacred” if Sunak leads them into the general election.
Clarke - a close ally of former PMs Boris Johnson and Liz Truss - is the first Conservative MP to break ranks and call for Sunak to go.
His dramatic intervention comes after a succession of opinion polls since Christmas showed Labour’s already-huge lead over the Tories getting even bigger.
Writing in the Daily Telegraph, Clarke said: “I know many MPs are afraid another change of leader would look ridiculous.
“But what could be more ridiculous than meekly sleepwalking towards an avoidable annihilation because we were not willing to listen to what the public are telling us so clearly?”
He added: “We have a clear choice. Stick with Rishi Sunak, take the inevitable electoral consequences, and give the Left a blank cheque to change Britain as they see fit.
“Or we can change leader, and give our country and party a fighting chance.”
Clarke was one of 11 Tory rebels who last week voted against Sunak’s flagship Rwanda bill, saying it would not work.
A Conservative Party spokesperson told The Sun: “This is a self-indulgent attempt to undermine the government at a critical moment for the country.
“He may claim to be helping the party but the only person he is doing any favours for is Sir Keir Starmer.”
Senior Tory MPs rallied round Sunak in the wake of Clarke’s criticism.
Former Brexit secretary David David said: “This is getting silly. The party and the country are sick and tired of MPs putting their own leadership ambitions ahead of the UK’s best interests.”
Former defence secretary Liam Fox said: “This is not the time for self indulgence and tribalism in the party.
“Those who have an agenda to destabilise the government in an election year should understand the consequences. Having been on the front bench for all 13 years in opposition, it is a miserable place. Be warned.”
Meanwhile, ex-home secretary Priti Patel said: “At this critical time for our country, with challenges at home and abroad, our party must focus on the people we serve and deliver for the country.
“Engaging in facile and divisive self indulgence only serves our opponents, it’s time to unite and get on with the job.”
Clarke’s comments come just a week after No.10 election guru Isaac Levido told Tory MPs to unite or lose the next election.
Speaking at a meeting of the backbench 1922 committeem, he said: “Let me be clear. Divided parties fail. It’s time to get serious - I am fighting to win this election, and I wouldn’t be here if I didn’t believe it was possible. We all need to be be fighting to win this election.”
Pat McFadden, Labour’s national campaign co-ordinator, said: “Labour will focus on serving the British people whilst the Tories form another circular firing squad.
“There are many good reasons for getting rid of this clapped out Conservative government and liberating the British people from endless bouts of Tory infighting is certainly one of them.”
Lib Dem deputy leader Daisy Cooper said: “It is utterly ludicrous that the Conservative Party is even discussing installing a fourth prime minister without even giving voters a say.
“The Conservatives are once again fighting like rats in a sack while families face soaring bills and an NHS crisis.
“People are sick and tired of this never ending Conservative Party soap opera. It’s time for Rishi Sunak to give voters the chance to put an end to this farce and call a general election.”