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 lyrics of a 1979 song by band XTC.
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.