Boris Johnson is facing more pressure over Downing Street parties as he attempts to cling to power, after receiving a questionnaire from the Metropolitan Police.

Senior Tory MPs claim that despite his determination to fight on it will be difficult for him to continue as prime minister if he is fined for breaching COVID lockdown rules.

Mr Johnson is one of more than 50 people across Downing Street sent a questionnaire by Met detectives and asked to give a “reasonable excuse” for attending alleged parties.

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2:49

PM has a week to answer questions

He is preparing to tell police he denies breaking the law. But even if he is fined, he is said to be ready to defy calls to quit, claiming he is a democratically elected prime minister with a huge mandate.

But the pressure is piling up. Three former Tory leaders have criticised the PM: Theresa May in the Commons on the day of the Sue Gray report; Sir John Major in a speech this week and Sir Iain Duncan Smith, who claims it will be very tough for Mr Johnson to continue if he is fined.

Warning of the threat to Mr Johnson, senior MP Sir Bob Neill told Sky News: “My consistent view has been that due process indicates that if anyone who is in a position that having made laws then breaches laws, I think it’s pretty difficult for them to continue.”

Sir Bob, a barrister who chairs the all-party Justice Select Committee, said the COVID rules were badly drafted and his committee was critical of them. But he added: “They are what they are.

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“Once they’re in place, even those of us who had doubts about them, once Parliament passed them, were obliged to stick by them.

“That of course means if you made the laws, and we volunteered to be in a law-making capacity, then we above all people just have to stick with them. Whether you’re a back bencher or whether you’re a minister. I think that’s the basic principle of respect for rule of law.

“If you have advanced calls and particular policy, then have been found to contradicted it or breached it, that makes your credibility pretty badly damaged. Never mind also the principle those of us who are elected to make the laws have to stick to them themselves.

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“You can’t just dismiss this and say it’s just a fixed penalty and it’s not important. Sticking to the rules, particularly when you’ve asked the rest of the population to stick to the rules – be that me as a local MP, be it anyone as a public health official, a minister – all of us have to be like Caesar’s wife. You have to be above suspicion.”

Lawyers claim police sending questionnaires – even though they have the same status as an interview under caution – is unusual and also point out that rules were toughened between the first alleged parties and the later ones.

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2:32

Johnson dismisses ex-PM’s claims

“It’s extremely unusual, although written questions under caution are sometimes sent in investigations,” Patrick Ormerod, a solicitor and expert on COVID lockdown laws, told Sky News.

“At the beginning of the pandemic there wasn’t an offence for organising large gatherings. There was an offence of gathering in a public place, an offence prohibiting leaving home without a reasonable excuse.

“So for example, the ‘bring your own booze’ 20 May party in 2020, the large gathering organisation offence wasn’t in place then. But by the time you get to December and the later parties, events or gatherings, that offence will be available and the 10,000 fixed penalty notice.”

On the prime minister’s claim that the May 20 gathering was a “work event”, Mr Ormerod said: “From what I understand there was a ‘bring your own booze’ email. It sounds like a social event rather than an event that was necessary for work purposes.

“The test is whether the event was necessary in terms of work. It’s not whether its work related. That’s not a defence in law.”

Having survived until parliament’s half term recess, the prime minister is at Chequers this weekend and he now has until next Friday to respond to the Met’s questions.