Green Party leader Zack Polanski defended his position Thursday after his party passed a motion defining Zionism as a form of racism and after he came second to Labour in a London by-election.

Former Green leader Caroline Lucas told the BBC that Polanski "should have been there and he should have voted" on the motion at the party's annual conference in Brighton. Polanski left the conference early to campaign in the Holborn and St Pancras by-election, where he finished behind Labour. The party's majority in the seat fell from 11,572 to 3,364.

The motion defined Zionism as an ideology supporting "an ethnonationalist Jewish state". Four of the five Green MPs opposed it. Lucas, who voted against, called the motion "dangerous" and "divisive" and said it was "flawed from top to bottom".

Polanski, who is Jewish, told reporters he grew up in a Zionist household but said his views changed as he "educated myself, learned, discovered". He said: "I believe that's clearly racist" in reference to what he described as genocide and displacement of Palestinians in Gaza. Israel has denied genocide allegations. The International Court of Justice is hearing a case brought by South Africa on the matter, though a ruling could take years.

Asked directly if he agreed Zionism is racism, Polanski said: "The displacement of the Palestinian people, the genocide, is totally racist." He did not give a direct yes.

Polanski attempted to reassure Jewish party members, saying "nobody will be excluded in the Green Party based on faith and religion". He declined to apologize, instead saying it was his job "to reassure people and to make sure that we bring our party together".

Ahead of the vote, Polanski had backed an amendment to protect Jewish members attending Zionist synagogues from discipline, but members voted it down. He also sought to change the reference to a single Palestinian state, arguing the Green Party should not decide what a future settlement looks like.

Polanski said he would continue as leader despite the by-election loss, stating: "Yes I've taken a loss tonight but our movement is so much bigger than one election or one motion."

Lucas told Newsnight she wanted to see what Polanski did next. She said: "I want him to show the leadership that was frankly missing on Sunday. I think the jury is out about whether he will do that."

Prime Minister Andy Burnham called the motion "completely wrong" and urged Polanski to reverse it.