Zack Polanski’s leadership of the Green Party remains under pressure in the wake of his defeat in the Holborn and St Pancras by-election.

Mr Polanski attempted to put a positive spin on his defeat in Thursday’s poll, saying the Greens had significantly increased their vote share in a usually safe Labour seat.

But the result has intensified criticism of his leadership in the context of a backlash to a controversial motion approved by the Green Party conference last weekend describing Zionism as racism.

Mr Polanski did not take part in the vote at the conference, and sought to distance himself from it on Friday saying he would only have supported an amended motion.

He told Sky News: “Ultimately, I agree with the substance of the motion. Where I disagree is the idea the Green Party should be deciding what the future of the region looks like.”

But his stance on the motion has brought calls for him to resign, with the chairwoman of the Jewish Greens, Vivien Lichtenstein, saying she was considering leaving the party and criticising his failure to take part in the debate as a “dereliction of duty”.

Separately, former leader Caroline Lucas told the BBC’s Any Questions programme that Mr Polanski should apologise for missing the vote and “for the hurt that has been caused to so many people”.

Although she stopped short of saying Mr Polanski should resign, Ms Lucas said there were “huge issues” in the party that he should help to resolve.

She also suggested there had been “entryism”, in which activists joined the Greens solely to back last weekend’s motion rather than because they supported the party.

She said: “I think we need to make our party structures much more robust so it’s not the case going forward that 1,000 members – some of whom, frankly, have been encouraged to join the party simply to push this motion through – 1,000 members should not be deciding a policy of such significance on behalf of 220,000 members.”

Mr Polanski has said it would be up to Green Party members whether he continues as leader, with a scheduled leadership vote due next year.

Asked how confident he was to be re-elected as leader, he told Sky News: “That’s going to be up to the members and that’s going to be up to the leadership election that happens next summer.

“What I’m experiencing, though, is an immense amount of people who are saying actually they’ve never been prouder to be a Green Party member.”

A YouGov poll published on Friday found 47% of people who voted Green at the last election thought he should remain leader, with only 22% saying he should go.

But among the wider public, only 20% said he should stay as leader and 34% said he should resign, with 47% telling the pollster they were unsure.