Downing Street spokesmen have said they will let it be known if Mr Johnson is issued with a fixed penalty notice, which would mean Covid laws had been broken.

However, they have declined to make the same commitment for any other senior Downing Street figure barring Simon Case, the Cabinet Secretary and head of the civil service.

It means that, should anyone other than Mr Johnson be fined for attending the gathering at the flat, the public will not necessarily be informed. The Metropolitan Police has said it will not be naming the individuals who are fined, in line with previous police guidance.

The force, which issued 20 fines over “partygate” this week, is also currently refusing to name which of the dozen events under investigation the fines have been issued in connection with.

The Telegraph has previously reported that Mr Johnson was preparing to argue, through his lawyers, that he attended the Downing Street flat gathering in a work capacity. The argument appears centred on the fact that the flat can be used both for work and socialising.

That was referenced in an update published by Sue Gray, the civil servant first tasked with looking into the allegations. She is understood to have received input from Mr Johnson.

The line in her update read: “The Prime Minister’s flat and the Downing Street garden are in close proximity to the offices and serve a dual office and private purpose.”

Interest in whether fines are issued over the flat gathering reflects the fact that, unlike the other events being investigated, it took place in Mr Johnson’s home. That makes it potentially more politically sensitive than others being investigated.