Former Cumbria MP Rory Stewart has quit the postponed Mayor of London race after facing "considerable challenges" in the delayed election.
The contest was originally due to take place on 7 May, but it was postponed for 12 months due to the coronavirus pandemic.
The 47-year-old, who was elected as the MP for Penrith and the Border in 2010, said: "It is with sadness that I have decided not to run in the postponed London mayoral election next year.
"I firmly believe London is the greatest city on earth - and it's courageous response to Covid-19 proves that more than ever.
He continues: "It would have been the honour of my life to serve the city as mayor, but while the considerable challenges of running as an independent were manageable for a normal race they are forbidding for an extended and delayed election.
"I want to thank my incredible team for all their efforts over the last year - I am so proud of you, of what we built from scratch, and of the values we championed.
"But even more than that I want to thank the tens of thousands of Londoners who engaged with my campaign, all the people I met on my walks - especially those who went on to invite me to stay - and everyone who shared my positive vision of all that London can achieve in the coming years."
Mr Stewart, who served in Theresa May's cabinet, was one of 21 Tory rebels who had the whip removed by Prime Minister Boris Johnson after defying party line in the Commons by backing a move designed to block a no-deal Brexit.