An aspiring Reform UK MP who advocated the use of concentration camps and said “you cannot be Muslim and British” has been suspended from the party.

The activist is one of a number featured in an ITV documentary, which will be broadcast in full on Sunday night and has already resulted in fresh questions about Reform’s vetting process.

The year-long investigation included undercover filming at an event referred to as Farage Fest, which was thrown by the party to thank volunteers who helped Nigel Farage get re-elected as the MP for Clacton during the recent byelection.

Those attending included Jason Powell, who said he had passed vetting checks to become a Reform parliamentary candidate, and who was recorded as saying: “Anyone who comes across on a boat, as an example, in the first week of election, build concentration camps, and everyone who comes across on the Channel, you’re in there.

“And that’s the first, the very first day … They just need to be rounded up, every single last one of them, and sent to those concentration camps, and then be given an ultimatum. Where are you from? Because you’re going back there. If you don’t tell us, you’re staying here for the rest of your life.”

Powell was also recorded as saying that he was “pretty hardline about Islam as well”, adding: “I would have disagreements, especially with, um, I mean, as far as I understand, Zia Yusuf [Reform’s home affairs spokesperson] is a Muslim. To me … you cannot be a Muslim and British … it’s like oil and water.”

Zia Yusuf is the Reform UK spokesperson for home affairs. Photograph: Carl Court/Getty Images

However, Reform has said that two other party figures who were elected as councillors in London and were recorded advocating the mass shooting of grooming gang members were not under investigation.

Adam Parkins was recorded as saying that if he was prime minister he “would quite happily do Sunday morning rose garden executions”.

David Kerr, a colleague of Parkins on Hounslow council in west London, said: “When it comes to like, yeah, grooming gang perpetrators, like, yeah dude, we just fucking line them up on the wall … line up a few thousand grooming gang perpetrators and machine gun them down.”

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Kerr and Parkins have been approached for comment. Powell later told ITV that his views were those of British people in general and are not extremist.

The Muslim Council of Britain, the UK’s largest Muslim umbrella group, said the suspension of Reform members meant very little given that other cases suggested people were sometimes quietly let back in again weeks later.

“The leadership of Reform UK must take full responsibility for the culture of hatred that they have allowed to fester and mainstream within their organisation,” said a spokesperson for the group.

Pressure meanwhile is also growing for the resignation of a councillor in London who was elected for Reform and who was recorded by the same ITV investigation, made with Hardcash Productions, allegedly calling for King Charles to be beheaded because he believed the king was secretly a Muslim.

Conservative, Labour and Liberal Democrat councillors in the south London borough of Bromley called on Dean Miller, who now sits as an independent, to stand down.

Speaking earlier in the week, Farage said that while Reform carries out vetting, “you can’t vet people for what they say in the future”.

In a statement released to the media this week, the party said: “Reform UK has improved and prioritised its vetting procedures, but from time to time a handful of individuals who do not share our values will initially make it through vetting.

“This is the same for all organisations. However good the vetting, one cannot vet a future comment.”

Undercover: A Year in Reform airs on 18 October at 10.20pm on ITV1, ITVX, STV and STV Player