In televised evidence that could determine his political future, committee chair Harriet Harman hit out at the “flimsy” assurances he was relying on for his Commons denials.

Mr Johnson also hit out at a Conservative colleague for raising a “completely ridiculous” assessment that his reliance on the advice of aides was a “deflection mechanism”.

In at times short-tempered exchanges, he called the inquiry “manifestly unfair” as he insisted he did not “recklessly” mislead the Commons.

Mr Johnson swore on the Bible to tell the truth before issuing an apology and adding: “I’m here to say to you, hand on heart, I did not lie to the House. When those statements were made, they were made in good faith and on the basis of what I honestly knew and believed at the time.”

Mr Johnson defended various rule-breaking events, including his birthday party for which he was fined, as being “necessary for work purposes”.

He said raising a toast surrounded by alcohol at a leaving do for departing communications chief Lee Cain was “not only reasonably necessary but it was essential for work purposes”.

The former prime minister said Mr Cain and Dominic Cummings had left their jobs “in very, very difficult and challenging circumstances”, and “it was necessary to steady the ship, it was necessary to show that there was no rancour, the business of the government was being carried on – that’s what we had to do, that’s what I had to do”.

Mr Johnson could be suspended as an MP and face a possible by-election if he is found to have committed a contempt of parliament by insisting rules were followed “at all times”.

He told the committee: “When I said the guidance had been followed completely [at] No 10, which is actually what I said, I was misremembering the line that had already been put out to the media about this event, which was ‘Covid rules were followed at all times’.”

Mr Johnson has insisted that he made his denials to parliament “in good faith” on the advice of his officials, which he now concedes turned out to be wrong.

Tory MP Alberto Costa said: “Some might see your reliances on the reported assurances you received as, and forgive me, as a deflection mechanism to prevent having to answer questions about your knowledge of these gatherings.”

Mr Johnson replied: “No, that would be a completely ridiculous assessment.”

Ms Harman, the Labour grandee chairing the Tory-majority committee, made a comparison of his witnessing gatherings and seeing a speedometer at 100mph but then denying speeding because someone assured him he was within the limit.

“Do you actually think we would be entitled to be a bit dismayed about the flimsy nature of this assurance?” she asked.

Ms Harman said it appeared his assurances “did not amount to much at all”.

Mr Johnson acknowledged he could have given a fuller explanation to MPs about his view on following coronavirus guidance in No 10.

“Perhaps if I had elucidated more clearly what I meant and what I felt and believed about following the guidance, that would have helped,” he said.

Mr Johnson, supported in the room by arch-loyalist Jacob Rees-Mogg, accused Ms Harman of having said in the past things that are “prejudicial” to his case.

He said if it was so “obvious” that rule-breaking was going on in No 10, as the committee argues, then it would also have been “obvious” to others, including Rishi Sunak.

He added that a claim by his former aide, Mr Cummings, to have raised concerns with the then prime minister was “unsupported by any documentary evidence” and “plainly cannot be relied on”.

“He [Cummings] has every motive to lie,” Mr Johnson said.

The former prime minister told the inquiry that if it is accusing him of lying, then it is also levelling the same charge at civil servants, advisers and MPs.

“I don’t think you seriously mean to accuse those individuals of lying and I don’t think you can seriously mean to accuse me of lying,” he said.

Mr Johnson is battling to avoid a suspension and a possible by-election that could follow if he is found to have committed a contempt of parliament.

In evidence published by the committee, officials denied telling him that no guidance was broken at mid-pandemic parties before he went on to insist all rules were followed.

Cabinet secretary Simon Case has told the inquiry that he was not aware of any officials giving Mr Johnson that assurance.

Jack Doyle, who was Mr Johnson’s communications chief when the Partygate story broke, said he did not tell Mr Johnson that all coronavirus guidance was followed.

And former principal private secretary Martin Reynolds said he warned against claiming that all rules had been followed before Mr Johnson issued a denial to parliament.

Mr Reynolds wrote to the committee saying he recalled asking Mr Johnson about a proposed line to say during Prime Minister’s Questions “suggesting that all rules and guidance had been followed”.

“I accepted this but questioned whether it was realistic to argue that all guidance had been followed at all times, given the nature of the working environment in No 10. He agreed to delete the reference to guidance,” Mr Reynolds said. But, on December 8 2021, Mr Johnson went on to tell the Commons “the guidance was followed and the rules were followed at all times”.

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