A documentary that lifts the lid on a “race science” network of far-right activists in Britain and its links to a rich American funder of eugenics research has been pulled from the London Film Festival (LFF) at the last minute due to safety concerns.

The organisers have taken the “heartbreaking decision” to cancel the planned screening of the “exceptional” Undercover: Exposing the Far Right this weekend due to fears about the welfare of audiences, staff and security working in the festival venues.

Havana Marking, the director of the film – which made headlines last week for identifying the backer of research into so-called race science and highlighting the racist views of former London mayoral candidate Nick Scanlon – has criticised the decision to pull the premiere as “a very unfortunate outcome”.

“I understand the festival need to look after their staff, but I am furious that our film has lost a planned theatrical release so late in the day,” she said. “We were told the LFF felt they could not show it due to security issues. I do feel, though, that the power of the far right is exaggerated, although their influence is clearly dangerous.”

Speaking to the Observer, Marking said she was worried about the climate of fear created by recent far-right riots in Britain in the wake of the killing of three children in Southport.

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“At least the film will go out on Channel 4 on Monday. And in fact, both Channel 4 and the British Film Institute, the body behind the film festival, have actually been incredibly supportive of this film.”

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The fly-on-the-wall documentary follows investigators from the organisation Hope Not Hate as they track down members of violent and bigoted far-right factions who are planning demonstrations and intimidation campaigns. It also unmasks the British far-right activist and former private school teacher Matthew Frost, also known as Matt Archer, and his connections to the Seattle-based multimillionaire Andrew Conru.