Nigel Farage’s Reform UK has reported Channel 4 to the elections watchdog, claiming the broadcaster used an actor as a “plant” in its undercover investigation into his campaign.

The broadcaster has denied that Andrew Parker, who was filmed using a racial slur to describe Rishi Sunak, was paid by or known to Channel 4 News before the expose on Mr Farage’s General Election campaign in the constituency of Clacton.

But Reform, in a letter to the Electoral Commission, said it was “wholly unbelievable” for it to be a coincidence and Channel 4’s piece “cannot be described as anything short of election interference”.

Channel 4 News has said it did not pay Mr Parker who was “not known” to the broadcaster and was “filmed covertly via the undercover operation”.

Separately, Mr Farage is embroiled in a dispute with the BBC, claiming the audience for Friday’s Question Time special was “rigged” and refusing to appear on the flagship Sunday With Laura Kuenssberg show unless the corporation apologises.

The row with Channel 4 stems from an undercover report on activists involved in Mr Farage’s bid to win a Westminster seat at the eighth attempt.

The footage showed Mr Parker using the racist term about Mr Sunak and suggesting migrants should be used as “target practice”.

Another canvasser described the Pride flag as “degenerate” and suggested members of the LGBT community are paedophiles.

In a letter to the Electoral Commission, Reform’s party secretary Adam Richardson said that it was “entirely evident that Mr Parker was a plant within the Channel 4 news piece”.

“It is wholly unbelievable that by complete coincidence Channel 4 were performing an undercover investigation and by chance were paired up to go canvassing with a man who was pretending to be someone else, using a false voice and saying almost exclusively racist and bigoted remarks.

“The Channel 4 broadcast has clearly been made to harm Reform UK during an election period and this cannot be described as anything short of election interference.”

The party said it was also planning to demand an investigation by broadcasting regulator Ofcom.

Mr Parker told the PA news agency he was “glad” that Channel 4 was being reported to the Electoral Commission.

He declined to say whether he had been paid to appear in the footage, instead telling the PA news agency that “it’ll all come out in the papers, what’ll come out is the truth”.

Mr Parker declined to comment further.

He has previously told PA his volunteering for Reform was separate from his acting job, adding that he had been “goaded” into making the comments caught on camera.

A spokesperson for Channel 4 News said: “We strongly stand by our rigorous and duly impartial journalism which speaks for itself.

“We met Mr Parker for the first time at Reform UK party headquarters, where he was a Reform party canvasser.

“We did not pay the Reform UK canvasser or anyone else in this report. Mr Parker was not known to Channel 4 News and was filmed covertly via the undercover operation.”

A spokesperson for the Electoral Commission said: “As Channel 4 News is a licensed broadcaster, its reports are exempt from the regime regulated by the Commission. Further, the Commission does not enforce the laws on defamation of candidates, which are a matter for the police.”

The watchdog said it had not received a letter from Mr Richardson, but added that “if we do then we will consider its contents”.

On the BBC Question Time special, Mr Farage was asked why his party “attracts racists and extremists”.

The Reform leader said he had “done more to drive the far-right out of British politics than anybody else alive”.

He said the audience was rigged and “these were not ordinary members of the public”, adding: “They hand-picked a prominent pro-Palestine activist & even a BBC TV director to attack me.”

Having taken part in a protest does not rule someone out from applying to be in the audience.

It is understood that there were no BBC staff members in the audience.

Mr Farage added: “I have just been invited to appear on Laura Kuenssberg. I’m refusing until the BBC apologises for their dishonest Question Time audience.

“Our state broadcaster has behaved like a political actor throughout this election. Reform will be campaigning vigorously to abolish the licence fee.”

A BBC spokeswoman said: “We refute these claims. Last night’s Question Time audience was made up of broadly similar levels of representation from Reform UK and the Green Party, with the other parties represented too.

“There were also a number of people, with a range of political views, who were still making up their mind.”