The former Boris Johnson aide who was behind a Facebook ad that broke the social network’s funding disclosure rules previously worked on a fake grassroots campaign pushing for a no-deal Brexit, the Guardian can reveal.

Alex Crowley, who left No 10 in September, oversaw the previous “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.

His new ad claimed that Labour’s tax plans would cost “an extra £214 each month” for everyone, and was posted under the name “Fair Tax Campaign”. But despite using clear political language, including the name of a political party, Facebook only became aware of the advert and removed it after being contacted by the BBC.

Crowley’s ability to evade the social network’s disclosure requirements raises questions about the ability of the company to enforce its own rules.

The new advert was posted without complying with Facebook’s requirements for political campaigning, which mandate organisers to prove they are resident in the UK, and disclose to readers who funds their advertising. It is the second time this week that the social network has had to take down adverts for breaking its rules, after it intervened in a government campaign that had originally run without the correct disclosure.

Facebook says it enforces its rules using a combination of human and machine moderation, but has not explained how this latest advert slipped through the net. The company told the BBC it had contacted the campaign “to provide clarity” on its rules.

Crowley, formerly a political adviser to Johnson during his time as mayor of London, later wrote a book about his experience of working with Crosby on political campaigns. He was revealed to have been involved in the Mainstream Network campaign by the Guardian in April this year.

In October 2018, the Mainstream Network campaign called on voters to “tell your MP to bin Chequers”, arguing for a harder Brexit than that negotiated by Theresa May. The campaign, estimated to cost £250,000, was the last major political campaign to launch before Facebook’s new rules requiring disclosure of funding, and MPs expressed outrage at the lack of transparency.

A Facebook advert placed by Mainstream Network in 2018. Photograph: Mainstream Network/89up /DCMS select committee

While Mainstream Network appeared to be an individual grassroots campaign run by a member of the public, it in fact shared an administrator who worked for Crosby’s CTF partners with a network of influential Facebook ad campaigns. Another former administrator on the network is now believed to be working with Conservative Campaign Headquarters’ digital campaign team.

At the time, Crowley said he had played a role in Mainstream Network because “some who disliked the result have been actively trying to overturn it”. He did not comment on his links to Crosby but said: “I wanted to see the result actually delivered and give a voice to that majority so the democratic result is honoured.”

Labour’s tax plans have not yet been published, so it is unclear what the basis for the £214 claim is. Crowley told the BBC that the claim was based on a report by the New Economics Foundation from August this year, but the only report from the NEF published in that month says nothing about Labour’s plans. It does contain a proposal to replace the personal tax allowance with a similarly funded “weekly national allowance”, but the proposal is not Labour policy.

But the removal of the advert may not be entirely negative for the Fair Tax Campaign. The campaign’s page, created three weeks ago, has received only 13 likes on Facebook in that time. The advert itself cost less than £500, and received between 35,000 and 40,000 views, according to Facebook’s ad library. But with the image now on the front page of multiple news sites, as well as leading BBC programmes including Today and the News at Ten, being banned has provided more awareness of its key claim than the group could ever have paid for.