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London mayoral candidate Rory Stewart has dropped out of the race after the coronavirus crisis forced a year's delay.
The former Tory cabinet minister said it was impossible to ask unpaid campaign volunteers to work for another 12 months.
The independent candidate said it had been an "agonising decision" for him to make.
The announcement comes on the eve of what would have been polling day.
In a statement Mr Stewart said: "I firmly believe London is the greatest city on earth - and its courageous response to Covid-19 proves that more than ever.
"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."
In March the government announced that the mayoral election would be delayed until May next year as the country prepared to go into lockdown.
After serving as International Development Secretary under Theresa May, Mr Stewart campaigned to succeed her in the Conservative leadership election last summer.
But after Boris Johnson became prime minister, Mr Stewart was one of 21 Conservative MPs expelled from the party for rebelling over Brexit.
The former MP for Penrith and The Border left parliament in October 2019 to run as an independent candidate.
Mr Stewart, 47, gained national attention through a series of campaigns that offered face-to-face debates with Londoners.
A cornerstone of his election effort were his daily walks across London were he would film interviews and discussions with Londoners he met.
His Come Kip With Me initiative, asking to stay a night in Londoner's homes to understand their problems, attracted national publicity.
In an interview with the Evening Standard Mr Stewart said he planned to write a book about politics and hoped to stay in public life.
He said would not re-join the Tory party and declined to endorse any other candidate for mayor.
Reacting to the news Shaun Bailey, the Conservative candidate, said: "Like me, he believes London needs new leadership.
"Rory brought some interesting ideas to the table and his unique campaigning style was a real breath of fresh air.
"I wish him all the best for the next chapter."