Historic comparisons were made with 1968, the last time a government lost three by-elections in a single day.

But while Rishi Sunak witnessed bruising defeats in the Tory heartlands of Selby and Ainsty and Somerton and Frome - suggesting he is on course to lose the keys to Number 10 - the party did hold on to Uxbridge and South Ruislip, the seat vacated by Mr Sunak's predecessor, Boris Johnson, in a wave of controversy over partygate.

The result has provided some small comfort to an otherwise downbeat party, with the prime minister saying it proved to Labour that the next election was "not a done deal".

On that, the Labour Party may well agree.

A blame game is underway after it failed to take what was logistically the easiest seat to win, with a 7,200 majority compared with Selby and Ainsty's 20,137.

Keir Starmer says Labour win a 'cry for change' - politics latest

On paper, Uxbridge was ripe for a Labour stealing, with changing demographics and a shrinking majority under the former prime minister that favoured the party.

But there was one spanner in the works that threatened the party's campaign from start to finish: Sadiq Khan's ultra-low emissions zone (ULEZ).

The policy, which will see motorists charged £12.50 a day if their car does not meet low emissions standards, has proven to be incredibly unpopular in Hillingdon in outer London, where it is due to be extended from the end of August.

Uxbridge win 'was going to be tough'

In a direct rebuke to the London mayor, Sir Keir Starmer told Sky News that his party lost the Uxbridge by-election because of the scheme's expansion.

"As for Uxbridge, we always knew that was going to be tough," he said.

"We didn't win Uxbridge in 1997, and obviously we knew that ULEZ was an issue. That's why we lost in Uxbridge.

"We all need to reflect on that, including the mayor [of London], but there's no taking away from the historic event that has happened here in Selby."

His words were echoed by deputy leader Angela Rayner, who also lay the blame for the result at ULEZ's door, saying: "The Uxbridge result shows that when you don't listen to the voters, you don't win elections."

Asked if ULEZ was the sole cause of Labour's defeat, one shadow cabinet member said: "It certainly looks like that. That's what you get when you have a cost of living crisis.

"We're going to have a lot to think about."

Reflecting on the campaign, a London Labour MP said it was the main issue that came up on the doorstep.

"People who were concerned about it were very strongly opposed," they said.

"Because of the cost of living crisis, because of the fuel crisis, because of the lack of assistance with a scrappage scheme from the government, this is not the time to introduce it and it should be postponed."

They also pointed out that the ULEZ scheme was in fact introduced by Mr Johnson, and that its expansion to outer London was ordered by the then transport secretary, Grant Shapps, under a financial arrangement for Transport for London.

"It does stick in my throat to hear the Tories crowing about defeating Labour using ULEZ when it was the Tories who introduced it in the first place," they said.

'Nobody will accept dirty water, why accept dirty air?'

Despite the pressure that is mounting on the mayor over the policy, Sky News has been told that Mr Khan has "no plans to change tack" with ULEZ, and his allies are urging him to stick to his guns.

And speaking to Sky News this morning, Mr Khan downplayed the chances of a Labour win in Uxbridge: "I have lived in London my entire life [and] this seat has never been Labour since I've been alive, including in 1997 in the Tony Blair landslide and the subsequent by-election a few months later.

"But I am quite clear though, the policy to expand the ultra-low emission zone is the right one. It was a difficult decision to take. But just like nobody will accept drinking dirty water, why accept dirty air?"

One Westminster insider supportive of the mayor said ULEZ had been used as a "deflection tactic" from a "poor campaign" - a charge that has been denied by those involved, who said the operation was "excellent" with thousands of activists out canvassing.

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The source said the decision by the Labour candidate in Uxbridge, Danny Beales, to publicly distance himself from ULEZ at a hustings was a "mistake".

"This is about how the dire national campaign allowed a by-election to become about a policy by a mayor, not about the policy itself.

"The Tories were desperate to make the election about ULEZ and Labour blundered into the trap by making the election about ULEZ. They need a clear out of their campaign."

It is always risky to judge the path of the general election by individual by-elections - but if there are any lessons from Uxbridge for Labour, it is that divided parties don't win elections.