Following the resignation of Owen Paterson over events that fall under the umbrella term of “sleaze”, support for the Conservatives has drained away to the extent that several leading bookmakers have now installed the Liberal Democrats as favourites to win on Dec 16.

The by-election will come at the end of a week that seems increasingly likely to decide whether Mr Johnson will face an imminent leadership challenge.

First, he must navigate a series of Commons votes on his “Plan B” Covid restrictions that already look likely to provoke the biggest Tory rebellion since the calamitous end of Theresa May’s premiership.

Tuesday’s votes in Parliament will show the extent to which his authority among MPs has been damaged by the events of the past month, and particularly the past week. It is difficult to overstate the anger in Tory ranks at the self-inflicted wounds over “Partygate”, “Flatgate” and the Owen Paterson affair, all of which are blamed directly on Downing Street.

Even MPs who have been loyal to the Prime Minister until now are describing his No10 operation as a “—-show” and a “shambles”.

Upwards of 50 Tory backbenchers have publicly expressed an intention to defy the whip and vote against Plan B next week, leaving Mr Johnson almost certainly having to rely on Labour votes to get the measures through.

The rebels include such prominent names as Sir Graham Brady, chairman of the 1922 backbench committee, former leader Sir Iain Duncan Smith and former Cabinet ministers David Davis, Liam Fox, Esther McVey, Greg Clark and John Redwood.

Party insiders believe that between five and 10 members of the Government will resign in order to vote against the measures. In Parliament this week, the Tory MP Will Wragg even went so far as to accuse the Prime Minister of bringing in the extra restrictions simply as a “diversionary tactic” to divert headlines away from his mounting problems.

None of which might matter if Mr Johnson was still as popular with the public as he was in 2019, when he defied gravity to pull off his 80-seat majority. But the problem for the Prime Minister is that his personal popularity, his sword and shield against his critics, appears to be on the wane.

Thursday’s by-election will be regarded as a referendum on his leadership – and the polls are not promising for Mr Johnson.

Earlier this week, a snap Savanta ComRes poll found a majority of respondents believed he should resign over the “Partygate” scandal. Then, on Thursday, two surveys of Westminster voting intention put Labour well in the lead: YouGov put Labour on 37 per cent, up four points on last week, with the Tories on 33 per cent, down three points; Survation put Labour on 40 per cent, up one point, and the Tories on 34 per cent, down two points. Since then, a third poll, by Focaldata, has Labour eight points ahead with 41 per cent to the Conservative Party’s 33 per cent.

A YouGov poll published on Friday showed the PM’s net favourability rating – which stood at +29 in April 2020 – had plunged to a new low of -42. Sir Keir Starmer’s net rating stands at -14, with the figure worked out by subtracting the “unpopular” responses from the “popular” ones.