Boris Johnson is under mounting pressure after his former chief adviser, Dominic Cummings, alleged that the prime minister lied to parliament about his knowledge of a garden party in Downing Street on May 20 2020 during the first Covid-19 lockdown.

Dominic Raab, the justice secretary and deputy prime minister, said on Tuesday that if it emerged that a prime minister had deliberately lied and it was “not corrected immediately, it would normally . . . be a resigning matter”.

Johnson told MPs last week that he joined the gathering of about 40 people — involving food and alcohol — for about 25 minutes and that he had thought it was a “work event”.

Cummings, who was working in Number 10 at the time, said on Monday he would “swear under oath” that the prime minister knew it was a party.

He claimed that Johnson had been informed about the event but shrugged off concerns that it broke the law. “He [Johnson] knew he was at a drinks party cos he was told it was a drinks party and it was actually a drinks party,” Cummings wrote.

Downing Street rejected this and said it was “untrue” the prime minister had been warned that the event could breach Covid laws. Johnson has authorised Sue Gray, a senior civil servant, to investigate a series of parties alleged to have taken place in Downing Street and elsewhere in Whitehall when the general public were unable to socialise in groups because of lockdown rules.

“As he [Johnson] 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,” a spokesperson said.

Raab told the BBC the two accounts were “irreconcilable” but that there had been a “categorical assertion . . . from Number 10” that Johnson had not known the event was a party.

However, when asked on the BBC’s Today programme whether a prime minister who lied to parliament should resign, Raab 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.”

Cummings offered further details on his Substack blog which contradict the prime minister’s version of events.

He said that Martin Reynolds, the principal private secretary in Downing Street, had sent out an email inviting 100 staff to “socially distanced drinks in the No 10 garden”.

Subsequently, a “very senior official” replied by email saying that the invitation broke the rules, Cummings added.

“The PPS went to the official’s office where they discussed it. The PPS declined to withdraw the invite. I told the PPS the invite broke the rules,” he wrote.

The former senior adviser — who rose to fame as the lead strategist in the pro-Brexit Vote Leave campaign — claimed that Reynolds told him he would “check with the PM”.

Cummings said he subsequently challenged the prime minister about the drinks party and said “you’ve got to grip this madhouse”, only for Johnson to wave aside the issue. He claimed that other eyewitnesses would back up his version of events.

Ian Blackford, leader of the Scottish National party in Westminster, said Cummings was a “key witness” and urged Gray to interview him as part of her inquiries.

Cummings himself was the focus of a major scandal about Covid-19 breaches in March and April 2020 when he claimed he drove to Barnard Castle in County Durham, north-east England, to test his eyesight during the first, most stringent, lockdown.

Opinion polls have highlighted the damage the “Partygate” row has done to the Conservative party’s standing with the public. Redfield and Wilton, a polling and research company, on Monday put Labour on 43 per cent, 13 points ahead of the Tories.

Only a small number of Conservative MPs have publicly called for Johnson to resign, with most choosing to await the Gray report.