His decision will put pressure on other ministers responsible for the criminal justice system including his boss, Dominic Raab, the Justice Secretary, who publicly backed Mr Johnson after Tuesday’s fines were announced.

Priti Patel, the Home Secretary, Kit Malthouse, the policing minister, and Suella Braverman, the Attorney General, have been advised against expressing public backing for Mr Johnson because of their “superintending” roles over criminal justice, even though all are strong supporters.

David Gauke, a former justice secretary, said he was “not surprised” by Lord Wolfson’s resignation, adding: “I think it’s a particularly uncomfortable issue for anybody in the Ministry of Justice or for that matter the law officers.”

It comes as sources close to the “partygate” investigation revealed that Mr Johnson faces a second fine after attending and giving a speech at a leaving party for Lee Cain, his outgoing director of communications, on Nov 13 2020.

It is understood he remained at the gathering for some time, making it more difficult for him to argue that it was a work event and that he had a “reasonable excuse” for being there.

However Simon Hart, the Welsh Secretary, indicated on Wednesday that Mr Johnson would not resign even if he was fined multiple times in the Metropoltian Police’s Operation Hillman investigation. The Prime Minister is reported to have attended six of the 12 events under investigation.

Mr Hart told Times Radio: “I don’t necessarily see the difference between one or two [fines], for example, the principle is the same.

“I personally don’t think that for people in public life – or any other walk of life, for that matter – that should necessarily be accompanied by another penalty, which is the removal of your job or similar.”