Large number of the 6,500 complaints about Ukip: the First 100 Days came via Britain First, which encouraged supporters to sign templated email

A significant proportion of the 6,500 complaints made about a controversial film imagining the early days of a Ukip government were the result of an orchestrated campaign by a far-right group known for its paramilitary style actions, the Guardian has learned.

Britain First, which leads “Christian patrols” through areas with high Muslim populations and has staged mosque “invasions”, encouraged supporters to sign a templated email to both Channel 4 and Ofcom to complain about Ukip: The First 100 Days.

Ukip docudrama: watchdog investigates after more than 6,500 complaints Read more

It is understood that a significant number of the more than 5,200 complaints received by Ofcom and the “vast majority” of the 1,300 sent to Channel 4 came via Britain First. The group itself has claimed to have sent more than 2,100, although the Guardian could not confirm those figures.

Ofcom said earlier on Monday it would open an investigation into the progamme, which depicted rioting after a fictionalised landslide Ukip victory in May this year, over claims it was offensive, misleading, and not sufficiently impartial.

The text of the letter drafted by Britain First was not available to those who signed it. A copy seen by the Guardian read: “I am disgusted by your attempts to brainwash the British public on the verge of a general election by airing a biased and ridiculous show called ‘100 days of Ukip’.”

It went on to claim that the programme incorrectly portrayed life under a potential Ukip government and amounted to “scaremongering”.

Britain First was set up by former British National party (BNP) member Jim Dowson, who left it in July 2014 complaining that it was attracting “racists and extremists”. The group’s members had developed the tactic of forcing their way into mosques to confront worshippers, which Dowson called “provocative and counterproductive”.

When asked if Ukip would distance itself from Britain First’s campaign, a party spokesman said: “It is nothing to do with us.” He provided the same response when asked if he would discourage Ukip members from taking part.

Ukip has sought to distance itself from Britain First on other occasions but the latter has claimed to have provided “support” at Ukip events in the past and vowed to do so again in the future.

The head of Britain First, Paul Golding, told the Guardian: “Ukip is rightwing and patriotic and we think they are being singled out for hostile media coverage in the runup to a general election.”

He said that the media needed to be fair in the period before an election. When asked if that applied to Ukip: The First 100 Days, which was fictional and aired before Ofcom’s purdah period, he said that no such programmes had been made about the other major political parties.

The complaints are in no way invalidated by their association with Britain First and will be considered by Ofcom.