A Labour official said voters who were not giving the party a hearing earlier in the year, were now listening again.

Moments after the declaration Polanski said he would stay on as party leader, telling broadcasters: “I’ve taken a loss tonight, but our movement is so much bigger than one election or one motion.”

Polanski’s defeat comes as Green Party divisions are spilling into the open after members passed a controversial motion declaring Zionism to be a form of racism at their party conference last weekend. 

One London councillor said the move had been  “electoral suicide,” and shortly after polls closed Thursday the veteran former MP Caroline Lucas told the BBC that Polanski, who did not attend the vote in Brighton, “should have been” there and should now “show the leadership that was frankly missing on Sunday.” 

Asked if Polanski should stand down, Lucas — a former party leader who was the first Green MP to be elected to the House of Commons — said: “I think let’s just see what happens in the coming days and weeks.”

But a senior Green local politician allied to Polanski, granted anonymity to discuss internal matters, said: “You can’t really force a coup” in the Green Party — pointing out leadership elections are not due for another year.

Polanski did not say if he agrees with party members that voted Zionism is racism when questioned at the by-election count, telling broadcasters:  “As I say, the displacement of the Palestinian people, the genocide, is clearly racist.”

This developing story is being updated.