A Reform councillor has accused ITV of “electoral interference” after undercover reporters joined its Portsmouth council election campaign as part of an undercover investigation into the party.

The ITV News Exposure documentary Undercover: A Year In Reform raised questions about Reform’s vetting procedures for candidates following undercover filming.

The party’s Hampshire leader George Madgwick was featured voicing support for the Portsmouth Patriots anti-immigration group. He subsequently issued a statement saying the undercover reporters potentially had an “impact on the result” of council elections.

ITV News has denied accusations of electoral interference. A spokesperson said the programme was produced in accordance with broadcasting regulations and was “in the public interest”.

Reform has been under pressure following the two ITV News reports, which showed undercover footage, gathered over the past year, showing some of Reform’s councillors and candidates using racist, violent and sexist language.

It comes two years after party leader Nigel Farage promised to root out “bad apples” from his party.

ITV News said it had reporters undercover with Reform UK candidates and councillors across England and Wales running up to the elections in May and four by-election campaigns this year and last.

The ITV Exposure documentary was produced by Hardcash Productions. The full 90-minute documentary – Undercover: A Year in Reform – is due to be broadcast on Sunday on ITV1 and ITVX.

A second ITV News report, external broadcast on Thursday showed footage of Madgwick admitting that the Portsmouth Patriots – members of whom were shown expressing far right views – were actively helping Reform to campaign in the run up to the council elections.

In a statement, he denied being a member of the group but said he had “supported their movement over the years from the sidelines”.

Madgwick said the two undercover reporters campaigned with the party in Portsmouth in March and April.

“It leads me to wonder whether their interference in an election is a potential criminal offence.”

“We have no way to know whether they posted flyers they were meant to. Or whether they gave us correct door knocking information,” he said.

He said they had campaigned in four seats, three of which the party narrowly lost

“Their involvement would have had an impact on the result,” he said.

He also said he was a “shocked and disgusted” the party had been “infiltrated” by the reporters.

“On a personal level I allowed these men in my home. I made them drinks. They met my dogs. It’s an entirely disgusting invasion of personal privacy.”

The Liberal Democrats took control of the council following the elections earlier this year, holding 22 seats, with Reform as the second-biggest party on 12 seats.

An ITV spokesperson said: “ITV’s programme Undercover: A Year In Reform was produced in compliance with the Ofcom Broadcasting Code, relevant laws, and its broadcast is in the public interest.

“All journalists were instructed to operate within this framework and did so under the supervision of the programme’s producers.”

The Electoral Commission said it sets out details of electoral offences which relate to campaigning for agents and candidates.

“Our Code of Conduct for campaigners provides a guide to what is, and is not, acceptable behaviour.”

“The consideration of any potential offences would be a matter for the police.”