The post comes less than three weeks after the death of Ann Widdecombe. The Reform UK spokesman for immigration and justice died after being hit on the head with a hammer 21 times at her home in Devon.
Joshua Kerry, a 28-year-old admin worker, appeared in court last week charged with the murder of the veteran politician.
The images were originally posted by a separate account that invited Mr Polanski to be a co-author of the post. A spokesman for Mr Polanski said he unshared the post once he was made aware of the offending image.
In the photograph, the activist, who was wearing a multi-coloured hat and sunglasses, posed for the camera and made a V-sign with his fingers. He was attending the “Green Party party”, a rally which promised to be “a day of love, joy, community and bangers”.
Zia Yusuf, Reform’s home affairs spokesman, said: “To do this just two weeks after our friend Ann was murdered is staggering. This is incitement to murder and it should be treated as such by the police.”
Reform has provided its MPs and senior politicians with round-the-clock security in the wake of Miss Widdecombe’s killing amid fears of a follow-up attack.
The party has agreed to provide full-time security to its representatives, many of whom have been followed, harassed or filmed while going about their daily lives.
However, it wants the budget for MPs’ security to be increased from £4m to £100m.
Hundreds of death threats a year
The party said threats of violence were now “baked into their routine” and that MPs needed to be able to work without “constant distraction by fear of aggression”.
Mr Farage has spoken out repeatedly about the level of abuse and threats of violence that he receives as leader of the Reform Party. He has received nearly 600 death threats this year alone.
Mr Farage resigned as Clacton MP last month amid a parliamentary probe into a £5m donation from Thai-based billionaire Christopher Harbone. Mr Farage, who said the money was to pay for his security, wanted the people of Clacton to be the judge of his actions. All mainstream parties have refused to field candidates for the Aug 13 by-election, which they have dismissed as a gimmick.
The parliamentary investigation into the donation is expected to begin again if Mr Farage is re-elected.
Lord Walney, the Government’s former adviser on political violence, said: “There must be serious consequences for Zack Polanski after he shared material implying the assassination of another politician.
“This would be sickening and unacceptable at any time but it is even more serious in the present environment in which a senior Reform member has been brutally killed. The Green Party must be barred from participating alongside mainstream parties if it fails to take action against its leader.”
A spokesman for Mr Polanski said: “After attending a public event, Zack accepted a request to collaborate on a post with several images.
“Once this image was brought to his attention, he immediately unshared the post. He unequivocally does not support this type of dangerous message.”
A spokesman for the Metropolitan Police said: “On Monday, 27 July, 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.”
Labour MP links terror attack to Reform
On Sunday, a Labour MP linked the Islamist terror attack at Berlin’s Pride festival to the rise of Reform UK.
Kate Osborne said Reform was part of an “escalation of hate” across Europe, which was a “catalyst for real-world physical violence”.
The MP for Jarrow and Gateshead East also singled out GB News, claiming the channel was “broadcasting content and language that attacks and vilifies the LGBTQ+ community”.