Keir Starmer called the prime minister “a man without shame” during furious exchanges in parliament as MPs prepared to vote on whether Boris Johnson should be investigated for lying about the Partygate scandal.

Addressing MPs for the first time since receiving a fixed-penalty notice for attending a party thrown for his birthday in June 2020, Boris Johnson spoke of his humility but said it had not occurred to him that the gathering was a breach of Covid rules.

In a fierce response, the Labour leader accused the prime minister of dishonesty and said he did not “respect the sacrifice of the British public”.

The senior Tory MP Mark Harper became the latest backbencher to call for Johnson to go, after hearing his apology, saying: “I no longer think he is worthy of the great office that he holds.”

MPs will vote on Thursday on a Labour motion that would trigger an investigation by the House of Commons privileges committee into whether Johnson misled parliament over a string of lockdown-busting parties in Downing Street.

Starmer urged Conservative MPs to seize the opportunity to get rid of Johnson and “bring decency, honesty and integrity back into our politics”.

In his statement to parliament, Johnson said he had broken the rules unwittingly. “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. That was my mistake and I apologise for it unreservedly,” he said.

Johnson added that the “hurt and anger” prompted by the row had given him “an even greater sense of obligation to deliver on the priorities of the British people, and to respond in the best traditions of our country to Putin’s barbaric onslaught on Ukraine”.

He said that he had been discussing the Ukraine conflict on Tuesday on a call with fellow world leaders, including presidents Joe Biden and Emmanuel Macron.

But Starmer called the apology “mealy mouthed”, and accused Johnson of being “dishonest”. When asked by the Speaker to withdraw that as unparliamentary language, Starmer said: “The prime minister knows what he is.”

Harper also rejected Johnson’s apology, saying: “I regret to say that we have a PM who broke the laws that he told the country to follow, hasn’t been straight about it, and is now going to ask the decent men and women on these benches to defend what I think is indefensible.”

Harper subsequently tweeted that he had already sent a letter of no confidence in Johnson to the chair of the 1922 Committee, Sir Graham Brady.

On Tuesday evening Craig Whittaker, a Conservative MP who has previously called for Johnson to resign, urged the prime minister to refer himself to the privileges committee to prevent taking Tories “to the brink”, in comments made to BBC Newsnight.

Despite calling for the PM to quit, Whittaker said he has not sent a letter to the 1922 Committee calling for a no-confidence vote.

However, other Tory MPs who have private reservations about Johnson said neither the apology nor move for a vote on an investigation by the privileges committee had shifted their view.

They remain in a “holding pattern”, waiting to see if any further fines are issued and hopeful the facts will come out in Sue Gray’s long-awaited report. One Tory backbencher said they were considering voting for the privileges committee investigation but that it would probably be treated as a confidence vote, meaning they would lose the whip and probably their seat at the next election for doing so.

Starmer cited the story of John Robinson, who wrote to the Guardian last week to say he could not hold the hand of his wife as she died, or hold a proper funeral for her, adding of Johnson: “Anger doesn’t even touch the sides of how I feel about this pathetic excuse for a man.”

Starmer told a subdued House of Commons: “If the prime minister had any respect for John and the millions like him who sacrificed everything to follow the rules he’d resign. But he won’t. Because he doesn’t respect John. He doesn’t respect the sacrifice of the British public. He is a man without shame.”

Labour sources said Starmer had returned from an Easter break with his family freshly enraged about the prime minister’s attempts to wriggle out of Partygate.

Johnson will be on a trade visit to India when Thursday’s vote is held, as he seeks to demonstrate to colleagues that he is getting on with the job of running the country.

In a signal that the Conservatives will whip their MPs firmly against the motion, the Cabinet Office minister Jacob Rees-Mogg cast doubt on the privilege committee’s suitability to investigate Johnson’s conduct.

He said it was a “distinguished body of the House of Commons but it’s chaired by a Labour party politician”, and added: “I’d bear that in mind.” The committee is always chaired by an opposition politician.

According to the ministerial code, ministers who “knowingly mislead parliament” are expected to offer their resignation.

The prime minister reassured MPs in December that “all guidance was followed” in Downing Street during the pandemic – a statement apparently contradicted by the Metropolitan police’s decision to impose a fixed-penalty notice.

Johnson has since argued that he did not knowingly mislead MPs because he had not believed rules were broken. Asked by the Tory MP Peter Bone: “Did you deliberately mislead the house at the dispatch box?” Johnson replied: “No.”

Another senior backbencher, Steve Baker, who has repeatedly criticised government Covid policy, suggested Johnson should be forgiven, stressing the need for “justice, and mercy, and humility” – though he asked Johnson for reassurance something similar could not happen again.

The prime minister and senior government officials could face further sanctions in the coming weeks as the Met continues its investigation into lockdown gatherings.

Once the Met has completed its work, the government has committed to publish the full report by Gray, a senior civil servant.