Tens of thousands of pounds are being spent on pro-Brexit Facebook adverts with little clarity on who is picking up the tab, despite the social network introducing new rules on the transparency of political adverts.

The biggest UK political advertiser on Facebook during the last week is Britain’s Future, a relatively obscure pro-Brexit group, which has spent £31,000 in seven days running more than 200 different adverts, mainly urging the public to write to their local MP and urge them to back Brexit.

However, there are no details on who is paying for adverts placed by the group, which is fronted by the former Two Pints of Lager and a Packet of Crisps scriptwriter Tim Dawson. Dawson did not respond to multiple requests for comment on whether he is subsidising its activities himself, whether he is relying on public donations, or whether there is another financial backer.

He is the only public individual associated with the group, which has spent £88,000 on pro-Brexit adverts in the last three months.

Many of the adverts placed by Britain’s Future use localised targeting to a greater degree than Vote Leave did in the 2016 EU referendum campaign and are designed to encourage members of the public to send messages to politicians who are making up their mind on which side to back in Tuesday’s crunch Brexit vote in the House of Commons.

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Facebook introduced new rules on political advertising transparency at the end of last year after a series of scandals associated with the 2016 US presidential election and the EU referendum. They require all political advertising to be labelled, to be placed in a public archive for seven years, and for those placing the adverts to verify they are living at a UK postal address.

However, the new rules do not force an advertiser to declare the ultimate source of the money for any political campaigns, meaning it is simple to place adverts with minimal transparency on who is ultimately picking up the tab.

Facebook’s Richard Allan said the company had taken “an industry-leading position” on ad transparency, going beyond what was required by law. “This includes a ‘paid for by’ disclaimer that requires an advertiser to accurately represent who is running that ad, and our ad library that archives political ads for seven years. We also publish a weekly report detailing ads run and top spenders across our platforms. This information gives everyone the ability to easily find out information about the pages running political ads and direct questions to them.”

He said the company welcomed the additional scrutiny. “This is exactly why we have introduced this industry-leading solution: to bring more transparency to political ads on the platform and encourage people to ask questions.”

Since the rules were introduced in October, by far the biggest political advertisers on Facebook have been anti-Brexit groups. People’s Vote has spent £220,000 pushing its message to the public, while the related campaign Best for Britain spent £150,000. The anti-Brexit groups have declared only partial accounts of their funding, although many big donors – such as the financier George Soros – have gone public with their funding.

Facebook records show that the UK government has spent almost £100,000 of public money promoting Theresa May’s Brexit deal in the last three months, while the Conservatives have spent tens of thousands of pounds pushing the prime minister’s vision for leaving the EU.

Facebook’s attempts to introduce background checks on political advertisers have had inadvertent side-effects for organisations that are not involved in day-to-day Westminster debate. The National Theatre found its ads for David Hare’s political play I’m Not Running were flagged for failing to include enough transparency details.