Legal experts said given that the alleged party in May 2020 took place inside the Prime Minister’s home it would be unlikely that he could face a fixed penalty notice for attending an illegal gathering.

But Adam Wagner, a human rights barrister and visiting law professor at Goldsmiths, University of London, said Mr Johnson could technically still be prosecuted as an accessory to other people’s crimes.

Meanwhile, The Good Law Project, a campaign group, announced that it was seeking a judicial review over the Met’s previous decision not to investigate reports of a Downing Street party on Dec 18, 2020.

Jolyon Maugham, the director of the group said: “You can have the rule of law, or you can defer to the powerful. But you can’t have both. Cressida Dick’s cat will know that multiple criminal offences were committed. It shames the Met, and ultimately all of us, that she refuses to investigate.”

Responding to an urgent question in the Commons, Michael Ellis, the paymaster general, said the Gray inquiry “will establish the facts and if wrongdoing is established there will be requisite disciplinary action taken”.

But he also said her investigation could be paused if evidence emerged of a criminal offence and the Met decided to launch its own inquiry.

Mr Johnson’s official spokesman said: “I think the Cabinet Office has been clear that they’re in contact with the police and obviously the police have the ability to start an investigation should they see fit to do so. That would then pause the work that has been done by the Cabinet Office team.”