Rory Stewart has pulled out of the race to become London mayor, saying it is too difficult to sustain a campaign extended by the coronavirus pandemic.

The former Tory Cabinet minister said on Wednesday, the eve of what would have been polling day, that he had made the “agonising decision” to abandon his ambition.

Sacked by Prime Minister Boris Johnson for rebelling over Brexit, Mr Stewart chose not to stand again as an MP for Penrith and The Border and instead set his sights on London.

The independent candidate asked voters to host him on their floors or sofas as he set upon a fact-finding mission in his bid to unseat the overwhelming favourite, Labour’s City Hall incumbent, Sadiq Khan.

But after Covid-19 forced the polling day to be delayed for a year until May 2021, a move Mr Stewart quickly called for, he wrote in the London Evening Standard that he has “decided not to stand”.

“There was no alternative,” he said of the decision to postpone the contest.

In such a grinding, costly, prolonged campaign the big party machines have the overwhelming advantage

“But sadly, this spelled the end of our dreams for London. Our independent campaign work could work as an insurgency, sustained by the free work of volunteers, over nine months. But not over 21 months.

“In such a grinding, costly, prolonged campaign the big party machines have the overwhelming advantage.

“I could not put our volunteers through another 12 months of campaigning in an environment completely changed by coronavirus and with the odds now so steeply against us.”

Mr Khan still faces a challenge for re-election from candidates including the Conservative’s Shaun Bailey.