Labour has said the London mayor, Sadiq Khan, is being targeted by an online “smear campaign” run by a former aide to Boris Johnson who has previously been involved with a “grassroots” campaign pushing for a no-deal Brexit. The party’s deputy leader, Angela Rayner, highlighted a menacing, anti-Khan attack ad that has begun to appear on Facebook, YouTube and elsewhere asking: “Do you want to use your vote to stop Sadiq Khan ruining our city?” The advert was put out in the name of the Fair Tax Campaign, but while it asks a series of questions about knife crime and transport against the sound of a heartbeat, it does not endorse a specific candidate in opposition to the incumbent mayor. It has been picked up by Nigel Farage, who retweeted it last week, attracting 2,000 retweets and 8,800 likes, and Facebook data suggests it has popped up 500,000 to 600,000 times. Fair Tax Campaign’s sole director, according to Companies House, is a former Johnson adviser, Alex Crowley, who helped run the Mainstream Network online campaign that called on MPs to “deliver Brexit” without a deal in 2019. After featuring an angry looking Khan pointing apparently at the viewer, the final images show a ballot paper with three options: “Sadiq Aman Khan, the Labour party candidate”, “someone with a plan”, and “someone you trust”. Rayner said the advert amounted “to dirty underhand tactics to try and smear Sadiq Khan” – and called on Khan’s Conservative rival, Shaun Bailey, to “immediately distance himself from these dirty tactics and call for this shadowy campaign to stop”. The advert appeared a little over a week ago, while a second from the same group accuses Khan of blaming “everybody but himself”. According to disclosures from Facebook, the Fair Tax Campaign has spent £13,730 in the past seven days, which Labour believes shows the beginning of a serious online effort. The Fair Tax Campaign solicits for donations online on its website, although it is unclear where the money spent so far comes from. The group did not respond to a request for comment when contacted by the Guardian. Groups such as the Fair Tax Campaign, which is campaigning in the mayoral election, but not directly acting for a candidate, are also subject to light disclosure rules. They do not have to register with or report donations to the Electoral Commission, although they are subject to a spending limit of about £30,000. Crowley was research director and then political director of Johnson’s London mayoral election campaigns in 2008 and 2012 and wrote a book about his early political career. He subsequently worked in Downing Street after Johnson became prime minister, but quit in September 2019 amid internal tensions. He was one of a number of individuals, linked to Sir Lynton Crosby’s CTF Partners, who helped organise the Mainstream Network no deal Brexit campaign, before running attack ads against Jeremy Corbyn in the run-up to the 2019 election. One of those anti-Corbyn ads was accused of breaching Facebook’s advertising rules because it did not carry an appropriate “paid for” disclaimer. Khan is well ahead in the polls to become London mayor and may win the election in a single round if he can achieve more than 50% in the first preference votes. Polling taken between 17 March and 20 March by Opinium has the incumbent on 53%. With Bailey struggling, on 28%, the attack ads appear to represent an attempt to reduce the mayor’s first preference votes in an attempt to force a second round run-off, where all the minor candidates are eliminated. That would almost certainly leave Labour and the Conservatives going head to head, although on current polling Khan would still be expected to win any run-off easily, when the election is held on 6 May.