Matt Zarb-Cousin said a "factional war" against the left had been "executed from the top down".
A prominent former Labour adviser has left the party to join the Greens in protest at Keir Starmer’s leadership.
Matt Zarb-Cousin, who was Jeremy Corbyn’s spokesman when he was leader, told HuffPost UK that left-wingers were “no longer welcome in the Labour Party” and urged others to defect.
He said he believed there was a “factional war” being waged against the left under Starmer’s leadership.
Zarb-Cousin, who also helped to run Rebecca Long-Bailey’s unsuccessful Labour leadership bid, also said reports that Labour would not give Corbyn back the party whip “sealed and confirmed” his decision to quit.
Explaining his decision to move to the Greens, which was first reported by Politico, he said: “Obviously I was attracted to the Greens’ policies on the climate crisis and proportional representation, which are the two most important things going forward for the country.
“Democratic socialists are no longer welcome in the Labour Party and there is a strong case for them to join the Greens and push for proportional representation, which will give our views and politics more influence.”
The Labour leadership has been accused of making sure left-wingers are not chosen as general election candidates — something Starmer’s team has strenuously denied.
But Zarb-Cousin said those around Starmer had been “blinded by their own rage to the left and have lost all sense of proportion”.
“There was some consensus around the Corbyn agenda and there was goodwill on the left to work constructively with Starmer,” he said.
“But there is now a factional war that has been executed from the top down.”
Zarb-Cousin suggested that others would follow his lead and move over to the Greens.
“I’m hearing that left-wing members have left and other people are thinking about it,” he said. “I think there will be more.”
There have been question marks over Corbyn’s future ever since he had the Labour party whip removed following his response to the Equality and Human Rights Commission’s (EHRC) report into anti-Semitism in the party’s ranks.
The EHRC investigation found Labour was responsible for unlawful acts of harassment and discrimination while Corbyn was at the helm, and identified “serious failings in the Labour Party leadership in addressing anti-Semitism and an inadequate process for handling antisemitism complaints”.
In response to the report’s findings, Corbyn said that while he accepted there was anti-Semitism in the party and that “one anti-Semite was too many”, he also believed the scale of the problem had been “dramatically overstated” by his opponents.
The former leader now sits in the Commons as the independent MP for Islington North — a constituency he has held since 1983 — but is understood to be holding out hope that he can stand for Labour again at the next election.
Those hopes appear to have been dashed following a report in The Guardian that said senior Labour figures would never permit him to stand for the party again.
One senior Labour figure told the newspaper: “Jeremy Corbyn is never getting back in. He would be toxic to our chances of winning back some of the seats we need to win back.”
Zarb-Cousin said Corbyn had been treated “terribly” and urged him to stand as an independent candidate in his north London seat.
“He has the support on the ground to do it,” he said.