Rory Stewart's support for Brexit makes him unfit to be mayor of London, according to one of his 2020 opponents.
Liberal Democrats mayoral candidate Siobhan Benita slammed Stewart for not backing the Letwin amendment in parliament yesterday.
The successful withheld parliamentary support for Boris Johnson's Brexit deal and effectively forced him to ask Brussels for a Brexit delay.
She said Stewart's support for Boris Johnson's Brexit deal showed he could never speak for London.
"It's staggering that someone running to be mayor of liberal, remain-voting London couldn't even bring himself to back an amendment which merely asked for extra time to scrutinise Boris Johnson's deal," she said.
"How can he claim to speak for London and talk about compromise when he backs an extreme version of Brexit that will so damage our capital city?
"I can't really understand why he's in this election and I think Londoners are wondering the same."
Stewart voted remain in 2016, but has since maintained that the result of the referendum must be respected.
However, the Penrith and The Border MP has also been a strong advocate against a no-deal Brexit.
The former international development secretary was thrown out of the Conservative Party last month after voting for the so-called Benn act, which sought to block no-deal.
Stewart said he voted for the government's deal yesterday to avoid the UK falling into "purgatory".
He said: "What really worries me is if in 10 years time we are stuck in a kind of purgatory where people are just digging themselves deeper and deeper into their positions.
"If this deal doesn't go through, we're going to be back into that purgatory of two opposing positions.
"I believe compromise can be - not a weak thing - but a strong thing."
Stewart will stand down as an MP at the next general election and run for mayor of London as an independent candidate.
He will be up against Benita, Green Party co-leader Sian Berry, Conservative London Assembly member Shaun Bailey and mayor of London Sadiq Khan in next May's election.