Sadiq Khan has been accused of “desperate gutter politics” over an attack website on Tory rival Susan Hall.
Labour has been targeting people in the capital with Facebook adverts linking to the site which claims to reveal the “shocking truth” about the Conservative London mayoral candidate.
The website by Labour’s London division makes a series of allegations against Ms Hall, including that she is “prejudiced against black Londoners”.
It also claims she “supports Donald Trump’s efforts to divide London’s communities” and “hates London’s diversity and says we are being ‘invaded’”.
But the Conservatives accused Mr Khan of “smearing” Ms Hall.
A Conservative Party source told the Telegraph: “Sadiq Khan is resorting to a desperate gutter politics strategy because he cannot defend his abysmal record.
“Just like Trump, he is smearing his opponent with misinformation and making false claims that the election is rigged. Londoners will see through this nonsense.”
It comes as Labour incumbent Mr Khan’s path to victory is far from certain in next May’s City Hall election.
He has faced a furious backlash over the expansion of the ultra low emission zone (ULEZ) to outer London.
A poll by JL Partners last month put him on 35 percent with Ms Hall just behind at 32 percent.
A London Labour spokesman said: “The mayoral election will be a close two-horse race with a choice between Labour’s Sadiq, who is building a better London for everyone and the out-of-touch Tory candidate.
“The Tories are changing the voting system by scrapping second preference votes used in every London Mayoral election since 2000 and introducing unfair new voter ID rules which unfairly penalise young people and minorities.
“Labour is urging Londoners to lend Sadiq their vote and not let the Tories do to London what they have done to the country.
“Sadiq will continue delivering for London – free school meals for every primary school child, record numbers of council homes and being tough on crime and its complex causes.”
