Boris Johnson has faced fury in the Commons after receiving a fine from the police for breaking lockdown laws.
Most Tory MPs rallied behind the PM as he repeatedly apologised for attending a birthday party in Downing Street in 2020, saying he did not think he was breaking the law at the time.
But Labour leader Sir Keir Starmer called his apology “a joke” as MPs from all parties criticised his behaviour.
Tory MP Mark Harper called on the PM to quit over his “indefensible” actions.
Labour has secured a vote on Thursday on whether a Commons committee should investigate if the PM misled Parliament.
Knowingly misleading Parliament is a resigning offence under government rules.
If MPs vote for an inquiry, the privileges committee – made up of seven MPs – could recommend sanctions, including an apology, a suspension or even expulsion from the Commons.
But the move is unlikely to succeed because the majority of Conservative MPs are standing by the PM, and they are likely to be ordered to vote against the Labour motion.
Asked directly by Tory MP Peter Bone if he had deliberately misled MPs in his past statements on Downing Street parties, Mr Johnson said: “No.”
Last week saw Mr Johnson become the first sitting prime minister to be sanctioned for breaking the law when he was fined by the Metropolitan Police, alongside his wife Carrie and Chancellor Rishi Sunak, over a birthday gathering for the PM in No 10 in June 2020.
The PM and others who partied in Downing Street during lockdown are widely expected to receive further fines, as the police continue their investigation.
But in his first Commons statement on his law-breaking, Mr Johnson said he wanted to get on with the job of “delivering on the priorities of the country at a difficult time”.
He told MPs: “It did not occur to me then or subsequently that a gathering in the Cabinet Room just before a vital meeting on Covid strategy could amount to a breach of the rules.
“I repeat that was my mistake and I apologise for it unreservedly.
“I respect the outcome of the police investigation, which is still under way, and I can only say that I will respect their decision-making and always take the appropriate steps.”
But Sir Keir called the PM’s apology “mealy-mouthed”.
The Labour leader accused Mr Johnson of offering “insulting” and “absurd” excuses for his Covid fine, and said he had eroded public trust in politicians.
Sir Keir also said the prime minister had been “dishonest” – but withdrew the remark after being rebuked by Commons Speaker Sir Lindsay Hoyle for breaking the rule that MPs do not accuse each other of dishonesty during debates.
The SNP’s Westminster leader, Ian Blackford, called the PM “a lawbreaker” and “a serial offender”, adding: “If he has any decency, any dignity, he would not just apologise, he would resign.”
And Liberal Democrat leader Sir Ed Davey said it was “profoundly damaging” to the UK to be “led by a man the public no longer trust and no longer have confidence in”.
Boris Johnson spoke to his MPs behind closed doors in the Commons on Tuesday night and he tried to do two things.
First, to deliver a call for unity by arguing the choice was really between the Conservatives led by him or the danger of a Labour government.
And second, that he still had a lot more to do to level up the country and get people off benefits and into work.
The prime minister said he had “genuine regrets” over Partygate, but it wasn’t a major part of his speech.
Although Mark Harper has called for him to go in public, privately even some of his critics say they won’t be backing Labour’s attempt on Thursday to refer the PM to the privileges committee to investigate whether he misled Parliament.
More challenges await the PM, including next month’s local elections.
But the fact that the police believe he broke the law hasn’t changed the feeling at Westminster that Ukraine and the cost of living crisis have – for now – taken precedence as issues and given him a get out of jail card.
Tory MPs lined up to back the prime minister, who made repeated references to the war in Ukraine in his statement.
But former Tory chief whip Mark Harper joined those calling for him to go – saying his actions had been “indefensible”.
He said the PM “hasn’t been straightforward” with MPs, adding: “I’m very sorry to have to say this, but I no longer think he is worthy of the great office that he holds.”
Mr Harper has submitted a letter of no confidence in the PM to Sir Graham Brady, chairman of the backbench 1922 committee, which organises Tory leadership contests.
A total of 54 letters would trigger a contest, but there is no running tally given of those handed in, with the number only revealed by Sir Graham when it reaches the threshold.
The PM later met his backbench MPs in the Commons as he tries to shore up support from within the Conservative Party.
He was seen arriving flanked by members of his cabinet, including Chancellor Rishi Sunak, and was greeted by the banging of tables as they welcomed him.
Mr Johnson was understood to have asked MPs if they would rather have him or Labour in power, adding: “We’re going to get on with our one-nation Conservative agenda.”