Reform UK have reported Green Party leader Zack Polanski to the police, after he shared an Instagram post including a photo of a man wearing a top featuring the name “Nigel” alongside an image of a guillotine.

One of several images showed a man in a black t-shirt with a large guillotine below the words “We’re only making plans for Nigel”.

Sharing a screenshot on X, Reform UK leader Nigel Farage said: “If I was to post anything as inciteful then I would expect to be arrested, and so should Polanski.”

A Green Party spokesperson said: “Once this image was brought to [Zack’s] attention he immediately unshared the post. He unequivocally does not support this type of dangerous message.”

Polanski had shared the photo with his 713,000 followers in a post containing several pictures of a Green Party event on Sunday, having accepted a request to collaborate with another account.

In a post on X, the Metropolitan Police said: “We received a third-party report relating to an image shared on social media. We take reports of this nature extremely seriously, and officers are conducting enquiries into the matter.”

The words “We’re only making plans for Nigel” are the same as the name and lyrics of a 1979 song by band XTC, external.

The Green Party spokesperson said: “Zack condemns the image on the T-shirt. He was tagged in a post with 20 images and clicked accept without realising there was more to it than he had seen.

“The T-shirt is clearly in poor taste, especially at a time of heightened fears for the security of public figures, having experienced multiple threats himself.

“He and the Green Party call on everyone involved in public discourse to show kindness and not encourage more division.”

The BBC understands Polanski has not been contacted by police.

Reform’s home affairs spokesperson Zia Yusuf said the image showed “a photo of man calling for the beheading of Nigel Farage”, which he claimed could be an “incitement to murder”.

Yusuf confirmed Reform UK had reported the post to police and will also report Polanski to to the monitoring officer at the Greater London Assembly, where the Green leader is an elected member.

“To do this just two weeks after our friend Ann was murdered is staggering,” Yusuf said.

“This is incitement to murder. It should be treated as such by the police.”

Prominent Reform member Widdecombe, 78, was found dead in her home in Haytor, Devon, on 9 July. A man, 28, has been charged with her murder.

Her death reignited a row about the security of political figures in public life, with this year marking the 10th anniversary of the murder of Labour MP Jo Cox and the fifth anniversary of the murder of Conservative MP David Amess.

Following Widdecombe’s death, Polanski called for all party leaders to get extra security.

Asked about Widdecombe’s death at the time, he told BBC Breakfast: “I am worried and I have been worried for a long time.

“I would encourage the government to provide me with extra security, but that is a choice for them.”

Andy Burnham also backed a serious review of MPs’ security earlier this month.

Reform UK’s Treasury spokesman Robert Jenrick had accused the government of a “dereliction of duty” over Farage’s security, after it emerged the party turned down a government-funded security offer last year.

The Home Office said Jenrick’s comments were “categorically untrue”, and that independent security experts, not ministers, held responsibility for deciding MPs’ security.

Sign up for our Politics Essential newsletter to read top political analysis, gain insight from across the UK and stay up to speed with the big moments. It’ll be delivered straight to your inbox every weekday.