Posters claiming "Israel is a racist endeavour" have been sighted in several spots around central London.
People said they saw the signs in at least three areas including Bloomsbury, Westminster and Waterloo.
The Met Police said it was investigating reports of flyposting in the Waterloo area and that offensive material would be removed.
It was not immediately clear who was responsible for the pictures but they are thought to have been put up in response to the Labour Party’s decision Tuesday to accept the International Holocaust Remembrance Alliance’s (IHRA) definition of anti-Semitism.
A group calling itself "London Palestine Action" tweeted several pictures of the signs, one with a caption that read: "1.8 million Palestinians, mainly refugees, locked in Gaza for being the wrong ethnicity. Israel is a #racistendeavour." The group is described as "taking creative action", but has not said they were involved with the posters. Euronews has reached out for comment.
London's transport service TfL said the posters were "flyposting" and therefore "an act of vandalism".
The company said it had instructed its contractors to remove the posters immediately.
It added the signs were "absolutely not authorised by TfL or our advertising partner JCDecaux".
Avi Mayer, a former Israeli Defense Forces spokesperson, wrote on Twitter that the posters were "horrifying".
Labour's announcement Tuesday concerning the acceptance of the IHRA document was met with some resistance — as some suggested the wording of examples could prevent criticism of the Israeli government's treatment of Palestinians.
The party at the same time approved a statement which “ensures this will not in any way undermine freedom of expression on Israel or the rights of Palestinians”.