Dominic Cummings, Boris Johnson’s former top adviser at 10 Downing Street, had “thrown a hand grenade” at the Prime Minister, the BBC said on Tuesday.

The comment was made on the BBC Radio 4’s Today programme by presenter Nick Robinson after Cummings had accused Boris of lying to Parliament, normally regarded as a resigning offence in British politics.

Later on Tuesday, Boris denied that he had lied to Parliament about a lockdown party, saying nobody had warned him the “bring your own booze” gathering might contravene Covid-19 rules.

Asked if he had lied to the public and Parliament, Boris told reporters: “No. Nobody told me that what we were doing was, as you say, against the rules ... I thought that I was attending a work event.”

He sidestepped questions about whether he would resign if proven he misled Parliament, saying only that he wanted to wait for the outcome of an internal inquiry.

Those with even cursory knowledge of how Parliament works will recall that in 1963, John Profumo, the patrician minister for war in Harold Macmillian’s Tory government, had to resign from the cabinet not because he had slept with 19-year-old Christine Keeler but because he had initially lied to the Commons by claiming he hadn’t.

Boris may find himself in the same position if it is established that he, too, lied to Parliament when he told the Commons that he had no advance knowledge of the party held in the garden of 10 Downing Street, on May 20, 2020, when Covid isolation rules were in force.

The ministerial code, governing standards of behaviour in office, states: “It is of paramount importance that ministers give accurate and truthful information to Parliament, correcting any inadvertent error at the earliest opportunity.

Ministers who knowingly mislead Parliament will be expected to offer their resignation to the Prime Minister.”

No 10 claimed that if anyone was lying, it wasn’t Boris but Cummings, who was sacked by the Prime Minister in November 2020, having previously played a crucial role in bringing him to power.

But since his dismissal, Cummings has been on a revenge mission to bring down his former boss.

Downing Street has denied Cummings’s claims, with a spokesman saying: “It is untrue that the Prime Minister was warned about the event in advance.

As he said earlier this week he believed implicitly that this was a work event. He has apologised to the House and is committed to making a further statement once the investigation concludes.”

The whole political establishment — indeed the whole country — is now waiting on Sue Gray, a senior civil servant, to deliver her report on what did or didn’t happen with “partygate”.

Cummings writes a regular blog almost entirely aimed at bringing down Boris.

On Monday, he wrote about the private discussions inside No 10 preceding the party on May 20, 2020: “Amid discussion over the future of the Cabinet Secretary (Simon Case) and PPS (Martin Reynolds, the prime minister’s parliamentary private secretary) himself, which had been going on for days, I said to the PM something like: ‘Martin's invited the building to a drinks party, this is what I’m talking about, you've got to grip this madhouse.’

“The PM waved it aside. I had told him repeatedly the PPS should be replaced, as had other competent officials who knew the whole structure needed a huge upgrade in personnel and management.

''Not only me but other eyewitnesses who discussed this at the time would swear under oath this is what happened.”

He said his account of the discussions leading up to the party on 20 May showed “the PM lied to Parliament about parties”. Two other former Downing Street officials told the BBC they remembered Cummings telling them that day he had warned the prime minister not to go ahead with the garden drinks party.

On Tuesday, when the justice secretary Dominic Raab, who also has the title of deputy prime minister, appeared on the Today programme, Robinson repeatedly asked him if Boris would have to resign if he was found to have misled parliament. Raab finally admitted: “Yes.”

He said: “He would normally, if it’s not corrected, if it’s lying, and deliberate... if it’s not corrected immediately, it would normally, under the ministerial code and the governance around Parliament, be a resigning matter. That is the principle. We uphold the highest standards of principles in public life. That is critically important.”

In a separate TV interview with BBC Breakfast, Raab said Cummings’s and Boris’s accounts were “irreconcilable” but the prime minister and No 10 had “made clear” he thought the drinks to be a work event.

Asked about how safe Boris was as leader, Raab said: “I’m confident he will carry on for many years and into the next election. Many have noted that the one senior politician who has conspicuously not taken to the airwaves to back Boris is the chancellor Rishi Sunak.