Scotland Yard was under mounting pressure on Tuesday night after it declined to launch a full-blown criminal investigation into "partygate" claims.
The Metropolitan Police said it would wait for Sue Gray to complete her independent report into alleged lockdown breaches in Downing Street before considering whether to investigate further.
But critics accused the force of dragging its heels over the matter despite growing evidence suggesting Covid laws had been broken.
Angry MPs on all sides hit out at Mr Johnson on Tuesday after a leaked email revealed that he had been among 100 people invited to a "bring your own booze" party in the Downing Street garden on May 20, 2020 – at the height of lockdown.
Witnesses at the event claimed Mr Johnson and his wife Carrie had been present, but a Downing Street spokesman refused to confirm or deny. Scotland Yard said it had made contact with the Cabinet Office over the matter but would not be launching its own investigation at this stage.
A senior Metropolitan Police source said the force would wait for the findings of the independent inquiry, which is now looking at allegations of eight separate gatherings by Downing Street staff.
The Metropolitan Police said it would wait for Sue Gray to complete her independent report
Credit: Peter Macdiarmid/LNP
But Sir Paul Stephenson, a former Met Commissioner, said the force would eventually have "little choice" but to launch a criminal investigation, while a senior Tory MP warned that any further delay would damage public confidence in the police.
Sir Paul said: "As always, I prefer the police to be doing things like investigating violence and drugs on our streets, but I suspect on this that there is little choice. Given the expressions of outrage, there will be an investigation."
The senior Conservative, a long-time ally of Mr Johnson, added: "You don’t need Sue Gray to know whether a party took place. I don’t think the Met can afford not to act."
Last month, the Met refused to investigate a Downing Street Christmas party widely reported to have been held at the end of 2020, when Tier 3 restrictions were in place.
A spokesman said at the time: "Based on the absence of evidence, and in line with our policy not to investigate retrospective breaches of such regulations, the Met will not commence an investigation at this time."
But a Scotland Yard source insisted the force had never ruled out investigating Covid breaches retrospectively.
What were the rules on December 18 2020?
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."