Allies of former Labour leader Jeremy Corbyn have called for him to be reinstated as one of the party’s MPs, arguing he is being “persecuted” by successor Sir Keir Starmer.
Unite union leader Len McCluskey described Sir Keir’s decision not to bring back Mr Corbyn as “vengeful”.
And former Labour chairman Ian Lavery called it “undemocratic”.
But Sir Keir said Mr Corbyn’s remarks on the scale of anti-Semitism within Labour had “undermined” trust.
Mr Corbyn was suspended from Labour at the end of October after a damning report by the Equality and Human Rights Commission on the party’s response to allegations of anti-Semitism during his tenure as leader.
The move followed Mr Corbyn saying opponents had “dramatically overstated” the scale of the problem within Labour for political gain.
The party’s ruling body decided on Tuesday to readmit Mr Corbyn as a member.
But this did not mean he would automatically be reinstated as a Labour MP, and on Wednesday Sir Keir decided not to restore the party whip to his predecessor – meaning he will continue to sit in the House of Commons as an independent.
Sir Keir promised to keep the situation under review, but allies of Mr Corbyn have urged him to change his mind immediately.
Mr McCluskey, whose Unite union is one of Labour’s biggest financial backers, said he was “astonished”, adding: “The continued persecution of Jeremy Corbyn, a politician who inspired millions, by a leadership capitulating to external pressure on party procedures risks destroying the unity and integrity of the party.”
What is the party whip?
MPs sitting in the House of Commons on behalf of a party are given what is called “the party whip”.
It allows them to represent their chosen party and stand for them in elections, as well as ensuring they receive a letter about forthcoming parliamentary business, detailing the position the party is taking.
Having the whip taken away is considered a serious punishment by a party.
While the MP can keep their seat in the House, they are classed as an independent and cannot run for the party in the next election, unless that whip is restored.
Mr Lavery, who served as Labour chairman under Mr Corbyn, leader from 2015 until earlier this year, told the BBC News Channel: “This is a strange way of rebuilding trust.”
He added: “Does this mean that, for the first time in Labour Party history, we have a leader who overrules who overrules democratic processes… because of what he sees as the right decision?”
Mr Lavery also asked: “Is this political persecution against the former leader?”
While many Labour MPs have supported Sir Keir’s decision, 28 MPs and four peers have signed a statement calling for a “swift reversal”.
The EHRC’s report found Labour had broken the law over its handling of anti-Jewish racism complaints by party members during Mr Corbyn’s tenure.
On Tuesday, Mr Corbyn attempted to clarify his position in public, saying that “concerns about anti-Semitism are neither ‘exaggerated’ nor ‘overstated'”.
But Board of Deputies of British Jews president Marie van der Zyl said Sir Keir had “taken the appropriate leadership decision”, adding Mr Corbyn had been “shameless and remorseless for what he has put the Jewish community through”.
Shadow justice minister Peter Kyle tweeted in support Sir Keir, saying: “We always knew tackling the cultural problems allowing anti-Semitism to exist in Labour would take time.
“We knew there would be moments that challenge and test us. The last 24 hours were unnecessarily difficult but Keir Starmer has acted authoritatively. We are moving forward.”
Fellow Labour MP Margaret Hodge – who is the parliamentary chair of the Jewish Labour Movement – also supported the move.
She said Mr Corbyn had “refused to himself accept the findings of the EHRC report, refused to apologise for his actions and refused to take any responsibility”, so it was right to withhold the whip.