Downing Street will be warned by senior Conservatives that a sea change is under way in the parliamentary party as growing numbers of MPs decide that Prime Minister David Cameron is a liability, amid a revolt over the issue of gay marriage.
As unfavourable comparisons were being drawn with former premier John Major, senior figures indicated that the chairman of the Conservative backbench committee, Graham Brady, was expected to receive further letters calling for a confidence vote. Brady, who is understood to have been sent a limited number in recent months, will have to call a vote if he receives at least 46.
One senior figure said: "This is worse than John Major. There was quite a lot of sympathy for him because of the Maastricht [anti-EU] rebels. He also listened. With Cameron it feels like this could be terminal - and will be so before the election."
Cameron's flagship gay marriage policy is set to deepen the rift. The debate has sharply divided lawmakers, with Conservative former minister Norman Tebbit stoking controversy by claiming gay marriage could result in a lesbian queen giving birth to an heir by artificial insemination.
"It's a perfect political storm. It couldn't have come at a worse time for Cameron," said Iain Dale, a prominent gay radio presenter and conservative blogger.
The bill passed a hurdle after Cameron was forced to strike a last-minute deal with the opposition to stop members of his own party. But the scale of the revolt, the second of its kind on gay marriage, is likely to damage Cameron's credibility anyway.
To compound Cameron's discomfort, he is also battling to squash a perception that he despises his own party members for being too conservative after newspapers quoted an unnamed aide as calling activists "mad, swivel-eyed loons".
Andrew Feldman, the co-chairman of the Conservative Party, fought off an attempt to hold an inquiry into allegations that it was he who provided the quote. Brian Binley, the Tory MP for Northampton South, agreed to withdraw his call for an inquiry at a meeting of the party board after Feldman won overwhelming backing. Feldman denies making the remarks which were reported, though not attributed to him, in The Times and The Daily Telegraph on Saturday.
Some Tories said the allegation that a senior Conservative had described activists as "swivel-eyed" marked a significant moment.
Additional reporting by Reuters