Boris Johnson has been told to resign by a senior Conservative MP for breaking coronavirus laws after repeatedly apologising and insisting he did not know he was breaching his own rules.

Former chief whip Mark Harper told Mr Johnson he is no longer “worthy” of being Prime Minister during a heated Commons debate on Tuesday, during which he was repeatedly told to quit.

Mr Johnson insisted he did not intentionally mislead Parliament with his earlier denials of rule-breaking parties, after MPs were granted a vote later this week on his alleged lying.

Addressing the House as it returned from its Easter recess, Mr Johnson apologised dozens of times for the “mistake” that saw him fined by police over the event in No 10 for his birthday in June 2020.

“Let me also say, not by way of mitigation or excuse but purely because it explains my previous words in this House, that 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,” he said.

“I repeat that was my mistake and I apologise for it unreservedly.”

Facing shouts of “resign”, Mr Johnson argued he feels an “even greater sense of obligation” to fulfil his duties as Prime Minister and to respond to the invasion of Ukraine.

Mr Harper revealed he had submitted a letter to the 1922 Committee of backbench Conservatives calling for a no confidence vote after branding Mr Johnson’s actions “indefensible”.

The Tory told the Commons: “I strongly support the Government’s actions in standing up to Putin’s aggression and helping Ukraine defend itself and our values, and it’s exactly at times like this that our country needs a Prime Minister who exemplifies those values.

“I regret to say that we have a Prime Minister who broke the laws that he told the country they had to follow, hasn’t been straightforward about it and is now going to ask the decent men and women on these benches to defend what I think is indefensible.

“I’m very sorry to have to say this, but I no longer think he is worthy of the great office that he holds.”

A no-confidence vote in the Prime Minister is triggered if 1922 chair Sir Graham Brady has 54 letters from Tory MPs. The number submitted is a closely-guarded secret.

Commons Speaker Sir Lindsay Hoyle approved an application from Labour leader Sir Keir Starmer and opposition MPs to hold a separate vote on Thursday on whether Mr Johnson intentionally misled Parliament.

Government sources insisted Mr Johnson’s trip to India is “definitely happening” despite it clashing with the vote, with one arguing it is “critical for jobs, trade, investment and diplomacy”.

Sir Keir accused Mr Johnson of being “dishonest”, language he was forced to retract after Commons Speaker Sir Lindsay Hoyle deemed it unparliamentary language.

But the Labour leader highlighted the case of John Robinson of Lichfield, who was unable to be with his dying wife during Covid restrictions, as he called for Tory MPs to act.

Sir Keir urged them to “put their conscience first, put their country first, put John Robinson first and remove the Prime Minister from office. Bring decency, honesty, integrity back into our politics.”

Labour is understood to be wording the motion to make Thursday’s vote about whether to refer Mr Johnson to the Committee of Privileges, which examines issues relating to contempt of Parliament.

The committee, the PA news agency understands, has the power to summon reports and documents.

It means that MPs could request to see the full version of senior civil servant Sue Gray’s inquiry into the Downing Street lockdown gatherings and any potential photographic evidence that exists.

A Labour source: “Any Conservative MP considering voting to block this investigation would be voting for a cover-up.”

Asked on Tuesday by Conservative MP Peter Bone if he “deliberately” misled the House, Mr Johnson replied: “No.”

Mr Harper argued in his letter to Sir Graham that “given we now know that the Prime Minister broke the law and there was systemic law-breaking in 10 Downing Street, it is difficult to avoid the conclusion that the Prime Minister has misled Parliament”.

In the Commons, the Prime Minister told Mr Harper that “I bitterly regret the event” but insisted he would “get on with the priorities of the British people”.

On December 1, the Prime Minister, addressing press reports about an alleged No 10 Christmas party in 2020, assured MPs that “all guidance was followed completely” in Downing Street.

But Mr Johnson has since been fined for the gathering on his 56th birthday, in which he was said to have been “ambushed” with a cake and up to 30 people attended, with the police investigation into other alleged breaches continuing.

The Conservative Party leader is alleged to have been at six of the 12 events being looked into by officers.

The Prime Minister, along with his wife, Carrie Johnson, and Chancellor Rishi Sunak, were last week issued with fixed-penalty notices (FPN) by police investigating claims of coronavirus lockdown breaches in No 10.