Jeremy Corbyn is coming under mounting pressure to sack Chris Williamson from the party as a key ally of the Labour leader said the MP “has to go”.

Jon Lansman, the founder of the grassroots Momentum group, said Williamson had not shown “one iota of contrition” after saying the party had been “too apologetic” in the face of criticism of the way it dealt with antisemitism within its ranks.

The Derby North MP – a strong supporter of Corbyn – was readmitted to the party on Wednesday after being issued with a formal warning by a national executive committee (NEC) antisemitism panel.

The decision produced a furious backlash, with more than 120 Labour MPs and peers led by the deputy leader, Tom Watson, demanding Corbyn step in to withdraw the Labour whip. Almost 70 Labour staff members have also written to express their anger at Williamson’s readmission.

They were backed by Lansman – one of Corbyn’s oldest allies – who accused Williamson of showing “contempt” for the panel’s ruling after he tweeted he had received an “avalanche” of support from grassroots members.

In response, Lansman tweeted: “This tweet reveals not one iota of contrition nor any acknowledgement of wrongdoing following a further formal warning from the Labour party for behaviour grossly detrimental to the party. Such contempt for the party’s verdict! He has to go!”

A Labour source insisted Corbyn was not involved in the party’s disciplinary processes or individual cases.

“It would be wholly inappropriate for a leader to pick and choose cases in the way that is being demanded,” the source said. “Several of the MPs who have signed (Mr Watson’s statement) have in the past argued against political interference.”

However, it has emerged that one of the members of the three-member panel that agreed to let Williamson back into the party with a warning has called for the decision to be reviewed.

Keith Vaz, the Leicester East MP, has written to Labour’s general secretary, Jennie Formby, expressing concern that the leaking of its ruling on Williamson to the media would mean its decisions in other cases could be subject to legal challenge.

He said to ensure the “complete integrity” of the process, either a new panel should be convened or all the cases it dealt with on Wednesday should be referred to the party’s disputes committee for reconsideration.

Williamson was suspended in February after video footage showed him telling a meeting of the Momentum group that Labour’s reaction to antisemitism allegations had led to the party being “demonised”.

He was also filmed saying he had celebrated the resignation of the MP Joan Ryan, who quit Labour in protest over the handling of antisemitism and bullying complaints.