Boris Johnson has faced a defection and a demand to quit from one of his most senior MPs during a dramatic day in Westminster, with even allies of the prime minister warning the current situation cannot go on. David Davis caused shockwaves when he told Johnson in the Commons: “In the name of God, go.” Less than an hour earlier, Christian Wakeford, the MP for Bury South, quit the Conservatives and joined Labour in fury at the Downing Street parties scandal. The prime minister vowed to battle on in No 10 and his supporters insisted he now had the breathing space for a fightback, with many MPs awaiting the outcome of the Sue Gray inquiry. But Johnson faces a growing clamour from Tory backbenchers to buy their support in any confidence vote by ditching a £12bn-a-year tax rise this spring. National insurance contributions are due to increase from April to fund health and social care and any U-turn could risk a showdown with the chancellor, Rishi Sunak. Johnson managed to get through the day without a confidence vote being triggered. Tory MPs estimated that as many as 30 letters may have been submitted of the 54 required, with more expected to come in after Gray, a senior civil servant, delivers her finding on alleged rule-breaking in Downing Street next week. The Guardian understands that members of the 1922 Committee of backbenchers are looking at shortening the period where a leader is immune from another vote of no confidence from 12 months to six. The same move was considered when Theresa May faced a no confidence ballot in December 2018 but survived. The rebellion has so far been led publicly by disillusioned 2019-intake MPs worried about the plunge in support in their seats. But sources said the next wave of letters was likely to come in from “One Nation” Conservatives from the centrist wing of the party if the Gray report is sufficiently damning of Johnson’s conduct. Tory MPs said Johnson was not out of the woods, with two ministers describing the current situation as a “shitshow”. One minister said he was reserving judgment until after the Gray findings but the prime minister did not have “carte blanche” to continue in post. The decision of many colleagues about whether to continue backing the prime minister may hinge on whether they think he will seriously harm the party’s chances in the local elections, he added. Numerous Tory MPs described to the Guardian a deluge of angry letters from constituents, ranging from 250 to 2,000 in one case – surpassing the anger at the lockdown-busting trips of the former No 10 aide Dominic Cummings. Johnson held meetings with MPs on Wednesday in an effort to persuade them to hold off from submitting no-confidence letters and tried to curry favour with his backbenchers by announcing an end to Covid regulations in England. However, backbenchers said he would need to do more and many remain undecided about whether he should continue in the job. One senior MP who is strongly supporting the prime minister said if he were to survive, he would need new senior staff in No 10 to “put him in a straitjacket”. The MP also advised a new “cabinet enforcer”, a reshuffle to remove disloyal cabinet members and a move to address the cost of living crisis by ditching the rise in national insurance contributions. “Is he mortally wounded? That is the question. He is wounded. But if anyone can turn this around, he can,” he said. A cabinet minister tried to claim Wakeford’s move had a “unifying” effect among Tories and could “draw a line under the whole thing” – but another said the prime minister’s position remained “precarious”. Senior Labour sources said they were in touch with other “very unhappy” Tory MPs about the prospect of potentially joining Labour, which would be a further serious blow to the prime minister’s authority. The defection happened just before a chaotic prime minister’s questions, with Wakeford crossing the floor to sit with Labour and blaming Johnson’s own “disgraceful” conduct. After a fierce set of exchanges between Johnson and Keir Starmer, the Labour leader said: “Doesn’t the country deserve so much better than this out-of-touch, out-of-control, out-of-ideas and soon to be out-of-office prime minister?” There was then silence in the Commons as Davis rose to tell Johnson that he had spent weeks defending him from “angry constituents” but that repeated reports about lockdown-breaching parties were too much. The former Brexit secretary and leadership contender said: “I expect my leaders to shoulder the responsibility for the actions they take. Yesterday he did the opposite of that. “So I will remind him of a quotation which may be familiar to his ear, Leo Amery to Neville Chamberlain: ‘You have sat there too long for all the good you have done. In the name of God, go.’” Asked by the Guardian afterwards what had motivated him to make an intervention, Davis said Johnson’s interview to Sky News on Tuesday was “not what I expect from a leader”. “Up until now I had been supporting him … but it’s not leadership,” he said. “Yesterday’s interview was an attempt to escape responsibility, not to shoulder it. And that is a test of leadership.” Junior colleagues are understood to have been pressing Davis to make a statement publicly calling for the prime minister to go, saying the situation needed a “big figure” to intervene. Johnson insisted in the Commons that he had no intention of resigning, while his press secretary afterwards told reporters that he would fight to carry on if any confidence ballot is triggered. The prime minister also revealed that Gray’s report is likely to be published early next week amid speculation it could come this week. With the party’s limbo continuing until then, many Tory backbenchers sounded despairing about the ongoing situation. “Where are the leaders? Where are the strategists to navigate our way through this?” said the Tory MP and former minister Tobias Ellwood. “Instead we have this blue on blue.”