Speaking after the by-election defeat, a loss which capped a week of political pain that included the biggest Tory parliamentary rebellion of his premiership over Covid passports, Mr Johnson tried to draw a line under recent blunders.

“I understand that what voters want us as the Government to be doing at all times is to focus on them and their priorities,” the Prime Minister said, as he took “personal responsibility” for the loss of the traditionally ultra-safe Conservative seat.

Mr Johnson also admitted failing to secure more media focus on the Government’s policies, saying: “I’ve got to put my hands up and say ‘have I failed to get that message across in the last few weeks? Has it been obscured by all this other stuff?’ Yes, I’m afraid it has.”

Attempts to move on from defeat backfire

However, Mr Johnson’s attempts to move on from the defeat backfired almost immediately as new allegations emerged about Christmas parties in Westminster last year.

The Guido Fawkes website reported that Mr Case attended a number of Christmas parties last year – a claim that sparked controversy given last week he was tasked with leading the probe into other alleged government Christmas parties.

Cabinet Office sources eventually confirmed that a virtual quiz was held on December 17 in which six of Mr Case’s private office staff took part together with drinks and food. Digital invites for the event in room 103 of the department in 70 Whitehall had the title “Christmas party!”.

December 17 was the day before the alleged No 10 Christmas party that forms the heart of the investigation.

Mr Case did not help to organise the quiz or take part, according to Cabinet Office sources, but did speak to those attending in his private office when he passed through – meaning he was aware of the gathering.

At the time of the event, London had just been placed under Tier 3 restrictions, meaning people were prohibited from mixing indoors, except within their household or bubbles.

Claims by Politico that Mr Case attended an event that involved more than a dozen people as well as drinks and food were categorically denied by Cabinet Office sources.

The reports triggered senior Labour and SNP figures to demand Mr Case step back from the probe into whether lockdown rules were broken.

The switch up could lead to delays in when conclusions are reached about any potential rule-breaking, opening up the possibility the probe could drag on into the new year.

It risks the row overshadowing attempts by the Prime Minister’s inner circle to try to refocus the Government on a policy agenda that it hopes will be delivered in time for the general election expected in 2024.

Hopes for Christmas ‘firebreak’ from damage

Mr Johnson’s close advisers hope the Christmas period can act as a “firebreak” from six weeks of damaging political rows – some self-inflicted – with a “let’s get back on track” moment in January to make clear to the public Downing Street is focused on driving through reforms.

Attempts to move on from weeks of Westminster-focused rows are expected to include the revelation of the long-awaited “levelling up” white paper, which will flesh out Mr Johnson’s much-discussed but vaguely understood flagship domestic policy drive, and possible changes in No 10 staffing.

A Cabinet Office spokesperson said: “Staff in the Cabinet Secretary’s private office took part in a virtual quiz on 17 December 2020. A small number of them, who had been working in the office throughout the pandemic and on duty that day, took part from their desks, while the rest of the team were virtual.

“The Cabinet Secretary played no part in the event, but walked through the team’s office on the way to his own office. No outside guests or other staff were invited or present. This lasted for an hour and drinks and snacks were bought by those attending. He also spoke briefly to staff in the office before leaving.”