The post-mortem has already started.

And it’s going to be a rough ride for Zack Polanski.

He wil be under intense pressure over the vote.

Not so much the vote result in the Holborn and St Pancras by-election.

But the vote passed just days before it at the Green Party’s conference in Brighton which declared that Zionism is racism.

Mr Polanski’s view on the deeply controversial motion is not fully known as he left his own conference, without taking sides, to go and join his party’s by-election campaign in north London.

It was a clear failure of leadership.

Many party members are more radical than MPs who are generally far more engaged in the day-to-day reality of politics and a number of senior Green figures voted against the motion but Mr Polanski was nowhere to be seen.

“It was a mistake not to show leadership on this, and it was a mistake not to be there to vote,” says one of the Green’s most respected figures, Caroline Lucas, its first MP who represented Brighton Pavilion for 14 years.

She slammed the motion as “dangerous” and “divisive”.

Pressed on BBC Newsnight whether he should stand down as party leader, Ms Lucas, herself a former leader of the Green Party, made clear with her scathing verdict that the jury was out.

“Let's just see what happens in the coming days and weeks,” she said.

“Obviously he has been completely taken up with this by-election.

“I want him to speak now about what's happened. I want him to show the leadership that was frankly missing.”

But he lost by a significant margin, having said the contest was ‘looking close’.

Labour’s candidate Sagal Abdi-Wali secured 45% of the vote (12,292), Mr Polanski 33% (8,928), a majority of 3,364 but with a 13% swing to the Greens, whose vote share was up by 22.4%.

The result shows that the Green Party is now a political force in London, especially after its success at the May local elections.

But it also reinforces the findings of recent polls that the Greens may come second in a string of constituencies in the capital, while failing to win any MPs.

So Mr Polanski’s leadership will be under the spotlight and it is not his first blunder.

He and his party soared in the polls earlier this year on the back of his style of brash, impulsive and outspoken leadership.

But Mr Polanski’s popularity has been falling, according to at least one poll, since he was slammed by Met Police chief Sir Mark Rowley in the spring in a row over the arrest made following the double stabbing in Golders Green.

The Green leader shared a post on social media suggesting the police used excessive force when detaining the suspect during the north London terror attack.

Hitting back at the criticism, Sir Mark wrote a public letter to Mr Polanski saying he was “disappointed” by the repost, which he labelled “inaccurate” and “misinformed”.

Mr Polanski, who contested the by-election in the face of the “Burnham bounce” for Labour in the polls, has vowed to carry on as Green Party leader and denied he had lost control of his party over the Zionism vote.

“I’ve got a party that was 55,000, 60,000 members when I was running for leader, it’s now 220,000,” he said.

“We’re a democratic grassroots party.”

True, but the words of Ms Lucas will be ringing in his ears.

He has weeks, if not days, to show he can deliver leadership including on highly divisive issues.