Sir Tony Blair has warned Andy Burnham not to enter a coalition with the Greens after the party was widely condemned for voting to adopt a policy defining Zionism as racism.

The former Labour prime minister hit out at Zack Polanski’s party, accusing it of crossing “a dangerous line”.

The move effectively “branded the majority of the (UK’s) Jews racists” at the same time as the Greens “harm the Palestinian cause—the very cause they say they support,” he said.

"There are those in my party, Labour, who might be tempted by the so-called progressive Red-Green coalition—an alliance between Labour and the Greens to unite the British left. But this vote should serve as a warning of the dangers such an alliance would bring," he wrote in the American website The Free Press.

Earlier this week senior Labour figures toldthat the prime minister should rule out any form of coalition with the Greens.

But the ex-PM is the most prominent Labour figure to publicly make the call.

Chief rabbi Sir Ephraim Mirvis has said that the motion, passed at Greens’ annual conference at the weekend, is a "threat to British Jews and to Britain”.

The chair of Amnesty International, Sandy Ruthven, left the party, telling the it was “failing to confront prejudice” in its ranks.

Israel also accused the Greens of legitimising “the destruction of the state of Israel” with a “clear antisemitic text”.

Mr Polanski, who did not vote, has been barred from entering Israel in response to the motion.

In the piece Sir Tony wrote: “If you care about the future of Britain, as I do, and if you care about the future of the peoples that live between the Jordan River and the Mediterranean Sea, as I do, this is a signal event.

“This weekend, the Green Party of England and Wales crossed a line. A dangerous line. One that it is unacceptable to cross.”

He added: “Since at least two-thirds of British Jews call themselves Zionists, the party has, in effect, branded the majority of the country’s Jews racists, while declaring that doing so cannot be antisemitic.

“If you care about the future of Britain, as I do, and if you care about the future of the peoples that live between the Jordan River and the Mediterranean Sea, as I do, this is a signal event.”

He warned that “when misguided or malevolently guided activists in Western countries campaign for Zionism to be delegitimized” they also “harm the Palestinian cause—the very cause they say they support.”

“Palestinians know Israel will continue to exist,” he said. “They know that if there were a one-state solution how unlikely it would be that such a state would be Palestinian. What’s more, resolutions like the one just passed by the Greens simply provide fodder for those on the far right in Israel, who also believe that Palestinians and Israelis can never live in peace.”

Ben Judah, who advised David Lammy at the Foreign Office until earlier this summer, said: “Tony Blair is right that Labour should not countenance a coalition with the Greens as long as this policy remains in effect.”

Adam Langleben, the former national secretary of Jewish Labour Movement and now director of the Progress think tank, said: “Tony Blair is absolutely right. This is a party that has abandoned the mainstream and adopted the antisemitic policies of the Soviet Union. It should be impossible to even entertain the idea.”

A recent YouGov MRP poll has Labour on course to be the biggest party but on 241 seats, well short of the 326 technically needed for a majority, suggesting the party could be in need of coalition partners.

A Green Party spokesperson said: "We won’t take lectures from a man who took this country into a catastrophic illegal war in Iraq that was justified by fabricated intelligence."