Chris Mason: Polanski faces greatest crisis of his leadership so far

In the aftermath of a by-election, the focus is usually on the winner. Not this time.

Make no mistake, the new MP for Holborn and St Pancras in north London, Sagal Abdi-Wali, has quite the story to tell.

She arrived in the UK as a child refugee from Somalia in the 1990s, grew up locally in Camden, became a councillor four years ago, the council leader in May and is now Sir Keir Starmer's successor as the MP.

But in the offices of Camden Council at three o'clock this morning, the focus of the press pack, me included, was on Zack Polanski, the leader of the Green Party of England and Wales, as we waited for the result.

This constituency has long been a safe Labour seat – the political home of the former prime minister and before that the former cabinet minister Frank Dobson. So a Labour victory here falls in the column marked "relief" for Downing Street rather than "jubilation".

Nonetheless, the Greens did very well in local elections in London in May, riding the crest of a political wave which began shortly after Polanski became the party's leader just over a year ago.

His failure to win here would have prompted questions from some about stalling momentum, but the party can and does point to the big swing to them since the general election and a result that was by a long way the second best ever result for them at a Westminster by-election.

Incidentally, in finishing third the Conservatives have beaten Reform UK for the first time in an English by-election since the general election. It seems likely that Reform's result was hampered by Restore Britain, who managed almost half as many votes as Reform did, in a constituency not very favourable to either party.

Follow live updates: Labour wins Holborn and St Pancras by-election, beating Green leader Zack Polanski

But the real reason for the nocturnal scramble of cameras, microphones and questions for Polanski had little to do with the by election – and everything to do with a vote at the Green Party's conference in Brighton last weekend.

Activists there passed a motion declaring Zionism as a form of racism.

The motion defined Zionism as an ideology supporting "an ethnonationalist Jewish state" and backed a "single democratic Palestinian state in all of historic Palestine", which includes modern-day Israel.

Polanski didn't take part in the vote because, he said, he chose to campaign in Holborn and St Pancras instead. And – until today at least – he has been repeatedly declining to do interviews, as the pressure on him as ratcheted up.

"He has created a monster he either can't or won't control," was how one Green figure put it to me - this person one among many left furious at what has happened.