Sadiq Khan is being targeted by an online 'smear campaign' by a former Boris Johnson aide, Labour has said.
Deputy leader Angela Rayner pointed to an online clip, which she said amounted to "dirty underhand tactics to try and smear Sadiq Khan."
And she called on his Tory rival, Shaun Bailey, to "immediately distance himself from these dirty tactics and call for this shadowy campaign to stop."
The menacing attack ad, which has appeared on Facebook and YouTube, asks: "Do you want to use your vote to stop Sadiq Khan ruining our city?"
It goes on to show the London mayor looking angrily at the camera, and names him as "Sadiq Aman Khan".
Nigel Farage retweeted the clip last week, and it is thought to have been shown to users on Facebook more than half a million times.
According to the Guardian, the clip was published under the banner of the "Fair Tax Campaign", which is run by former Johnson aide Alex Crowley.
The Mirror revealed last month that Mr Crowley's campaign group had spent more than £63,000 on around 100 anti-Jeremy Corbyn ads ahead of the 2019 general election.
In 2019 it was reported Mr Crowley, who left No10 just a month before the election was called, had worked on a fake grassroots campaign pushing for a no-deal Brexit.
The Guardian reported Mr Crowley had overseen the “Mainstream Network” Facebook campaign alongside employees of the lobbying firm run by Sir Lynton Crosby, the Australian political strategist who helped run three Conservative general election campaigns.
In November 2019 one of the Fair Tax Campaign’s ads was banned by Facebook, after the campaign failed to properly declare it as a political message.
Approached by the Mirror, Mr Crowley did not deny being behind the campaign.
He said: "The only advert Labour should be worried about is what Khan’s record of complete failure in London says about their ability to govern."
