The Metropolitan Police has said it will be taking no further action after Green Party leader Zack Polanski re-posted a photo of a man wearing a T-shirt with an image of a guillotine and the slogan “we’re only making plans for Nigel”.

In a statement, the force said it did not believe sharing the picture constituted an offence.

However, it added that the wearing of the T-shirt should be recorded as a crime and had referred a crime report to Avon and Somerset Police, who cover Bristol where the photo was taken.

Polanski previously told the BBC he had made an “inadvertent mistake” by re-posting the image.

In response to the Met’s decision, a Reform UK spokesman said: “Once again we see that we are living in two tier Britain.”

Reform UK reported Polanski to the police on Monday after he shared the Instagram post.

The Green Party of England and Wales leader had shared the photo, which he said was the fifth image in a series of 20, having accepted a request to collaborate with the Instagram account of a photographer at an event.

Speaking to BBC Newsnight, Polanski said: “If I’d done it deliberately I’d be apologising but because this was a mistake I think the important thing is that I point out this was a mistake, that I also make sure that I condemn political violence, and also crucially I make a commitment to be better on social media.”

Announcing the decision to take no further action, a Met Police spokesperson said: “We absolutely understand the reaction to this image being shared online, particularly in the context of increasing threats towards politicians.

“We take the safety and security of those in public life, including politicians, very seriously.

“Where we identify threats we take appropriate action and where there is evidence of offences we will investigate.

“While there will be people who will argue that those who shared the image in this Instagram post should be subject to criminal action, our review has concluded that on this occasion, the threshold is not met.”