The single biggest known British political advertiser on Facebook is a mysterious pro-Brexit campaign group pushing for a no-deal exit from the EU. The revelation about Britain’s Future, which has never disclosed the source of its funding or organisational structure, has raised concerns about the influence of “dark money” in British politics.

The little-known campaign group has spent more than £340,000 on Facebook adverts backing a hard Brexit since the social network began publishing lists of political advertisers last October, making it a bigger spender than every UK political party and the government combined.

However, there is no information available about who is ultimately paying for the adverts, highlighting a key flaw in Facebook’s new political transparency tools.

The sophisticated campaign includes thousands of individual pro-Brexit adverts, targeted at voters in the constituencies of selected MPs. The adverts urge voters to email their local representative and create the impression of a grassroots uprising for a no-deal Brexit. The MPs then receive emails, signed by a “concerned constituent”, demanding a hard Brexit. The emails do not mention the involvement of an organised campaign group.

Britain’s Future’s public presence contains links to just two individuals: an ex-BBC Three sitcom writer turned journalist, and, indirectly, a former BNP candidate who lives on a farm called Rorke’s Drift in the Yorkshire dales.

The site’s public face is Tim Dawson, who created the sitcom Coming of Age while still in his teens before going on to contribute to Two Pints of Lager and a Packet of Crisps. In recent years he has stood for election to Manchester city council as a Conservative candidate before last year taking control of Britain’s Future.

Dawson’s pro-Brexit campaign group has spent more than a third of a million pounds on targeted Facebook and Instagram adverts in just a few months, including more than £50,000 last week alone, urging voters to email their local MP and tell them to get Britain out of the EU. An further unknown sum has also been spent buying up adverts alongside Google search results related to Brexit, suggesting that the total amount spent by his organisation on online campaigning could be much higher.

Throughout all this, Dawson, who these days makes a living from writing occasional pieces for the Daily Telegraph and the Spiked website, has declined to comment on the source of his funds, other than to tell the BBC that he was “raising small donations from friends and fellow Brexiteers”. There was no answer at his flat in Manchester and he has repeatedly declined to answer questions on how he has access to levels of funding that dwarf many high-profile campaigns.

According to its Facebook page, there are at least five individuals involved in the administration of Britain’s Future, although there are few clues as to who they are. Its “About Us” page contains a map centred on a remote building in the Yorkshire Dales north of Harrogate. This is Rorke’s Drift farm, named after the 1879 battle in South Africa where a small group of British soldiers made a successful last stand against thousands of Zulu warriors, an incident later depicted in the Michael Caine film Zulu.

The farm is home to Colin Banner, a former British National Party candidate. When contacted by the Guardian, he insisted that he had no knowledge of Dawson, was not aware of Britain’s Future, and was not involved in placing the adverts.

In a rare statement, Dawson declined to answer questions on funding or who was behind Britain’s Future. He said it was pure coincidence that his website was pointing to the remote home of a one-time BNP candidate and thanked the Guardian for bringing it to his attention.

“Britain’s Future has never associated with, nor would it ever associate with Colin Banner, or any BNP member. I have never met with, spoken to, or associated with Colin Banner, or any BNP member, nor would I want to. To state otherwise would be untrue.

“Designing the website required selecting a point on the map of the UK. The coordinates were randomly selected so the map of the UK would display centrally on the webpage. It was solely a design decision.

“The purpose of Britain’s Future is to represent the views of 17.4 million people who voted to leave the European Union – regardless of background. This is about delivering on the result of the referendum.”

No law is being broken by Britain’s Future’s campaigning. Outside of an election period, it is legal for any individual or campaign group to pay to promote political material without declaring where the funds come from. Britain’s Future is not a political party and does not appear to have any intention of putting forward candidates in elections, so is not regulated by laws requiring large political donations to be publicly declared.

Even the anti-Brexit People’s Vote campaign for a second referendum, backed with financing from the billionaire George Soros, has spent less on Facebook than Britain’s Future. Its website is essentially a personal blog on arguments for Brexit, with a discreet PayPal button soliciting donations.

Under Facebook’s transparency rules, a representative of Britain’s Future would have been required to provide a valid UK postal address before placing political adverts, but this information was not made public. There are no checks on the ultimate source of any funds.

Facebook said it was only thanks to its new political ad transparency tools, introduced after the EU referendum and soon to be rolled out across the UK, that it was possible to see the extent of political advertising placed by Britain’s Future. There is no equivalent database for Google, Twitter or other online advertisers.

Dawson previously stood as the Conservative council candidate in Manchester’s Hulme ward last year and finished a distant sixth. He gave an interview to Country Squire Magazine, explaining that he had recently embraced politics after becoming exasperated with the leftwing bias of the BBC: “There are lots and lots of Conservatives in this country and they deserve to be represented in our cultural landscape.”

Last month, a report from the Department for Digital, Culture, Media and Sport warned that electoral law was out of date and vulnerable to manipulation by hostile forces, and that the need to update it was urgent.