No 10 confirmed on Friday evening that the Prime Minister had received the legal form from Metropolitan Police officers, and said he will “respond as required”.

The move means Mr Johnson will have to provide a credible reason as to why he was at events held during coronavirus restrictions or face a fine.

A No 10 spokeswoman said: “We can confirm the Prime Minister has received a questionnaire from the Metropolitan Police. He will respond as required.”

Boris Johnson has been contacted by police

Earlier it, the Chancellor appeared to suggest that he does not expect to be asked to fill out a police questionnaire about his involvement in a No 10 event that possibly broke Covid rules.

Scotland Yard has said it will be sending formal legal questionnaires by the end of the week to more than 50 individuals, from whom officers involved with Operation Hillman – the probe into so-called “partygate” – wish to hear, regarding the 12 events they are investigating.

Rishi Sunak has previously confirmed that he was at one of the events subject to Metropolitan Police inquiries – a birthday do held by Downing Street staff and the Prime Minister’s wife Carrie Johnson for the Prime Minister on June 19 2020.

However, the Chancellor has insisted that he had been present at the Cabinet Room on the day in question for a routine meeting to discuss the pandemic situation.

On Friday, Mr Sunak told Sky News he had not received a questionnaire from the Met and, asked whether he expected to receive a form to complete, he replied: “No… well, I don’t know.”

The Treasury chief also said he did not think he had broken Covid rules.

When asked last week about the birthday celebration, at which one minister described the Prime Minister as being “ambushed by cake”, Mr Sunak said he went to the Cabinet Room as he had for “100, 200, God knows how many other Covid meetings”.

Pressed on Friday about whether he still has confidence in Mr Johnson, the Chancellor added: “The Prime Minister has my total support.”

It comes after Mr Johnson faced a verbal mauling by one of his Conservative predecessors, while the Prime Minister was touring European capitals on a diplomatic mission to calm the tensions between Russia and Ukraine.

Former prime minister Sir John Major used a speech on Thursday to accuse Mr Johnson and No 10 officials of breaking lockdown laws and concocting “brazen excuses” about the partygate affair.

The Prime Minister was defended by a junior minister during the morning broadcast round, with Robert Courts telling LBC he understood that the claims of rule-breaking had “caused great public upset” but that Mr Johnson had “made the right decisions at the right time throughout this pandemic”.

The Metropolitan Police faces its own turmoil after Commissioner Dame Cressida Dick was forced out in a row with London Mayor Sadiq Khan about how to improve the force’s performance and reputation.

Dame Cressida quit on Thursday, saying she had come to the conclusion that the mayor had lost confidence in her.

But Mr Courts said the police investigation into what went on at the top of Government will “not be affected” by the change of Met chief.

He told Sky News: “Police are very used to handling matters where there are different parties involved.

“They are totally independent and I have absolute confidence in their ability to operationally carry out an investigation.

“So they will do that and they will do that in a way that is independent – that will not be affected by the role at the top of the Met.”

Boris Johnson contacted by police in partygate probe