Frank Field has in effect resigned from the Labour party, according to a report delivered by its general secretary.

Jennie Formby told the party’s disputes panel on Tuesday that the MP for Birkenhead could no longer be a member because he has resigned the party whip.

Field previously said he planned to challenge any attempt to remove him from the party. However, the panel did not challenge Formby’s statement saying he had in effect already left, sources said.

The dispute comes amid claims that Labour faces an “existential threat” from MPs who are discussing the establishment of a separate party.

Senior Labour figures, including John McDonnell, the shadow chancellor, had expressed hope the row could be solved without further political damage. Field had said McDonnell suggested he should discuss antisemitism and bullying claims with Formby.

Field resigned the whip last week, meaning he is no longer bound by Labour rules or forced to vote in line with the party. He said he acted because Labour risked becoming a “force for antisemitism” and had allowed a “culture of nastiness, bullying and intimidation” to develop.

On Friday, he held a meeting with the chief whip, Nick Brown, where he was told he would have to rejoin the party if he wished to still be a Labour member. Field, who chairs the work and pensions select committee, said Brown told him he had two weeks before he would be removed.

Labour disputed that Field had been given time to change his mind. “The chief whip conveyed to him that the rules state resigning the whip means resigning from the party. That happens immediately – there’s no two-week cooling off period,” a source said.

Formby, the party’s most senior official, spoke to the disputes panel on Tuesday morning and delivered a briefing on Field’s status and his meeting with Brown. She endorsed previous statements from Labour claiming that Field had automatically opted to resign by his actions.

Field told the Guardian on Tuesday he would challenge in the courts any attempt to revoke his membership, which he will have to maintain if he is to stand again as a Labour MP.

“I have not been told about any discussion inside the party about my membership. All I know is that this is not the views of the lawyers I have consulted, so we will see. I will talk to the lawyers again tomorrow but this obviously won’t go unchallenged,” he said.

Field, who has been approached by the law firm Mishcon de Reya, has claimed that several peers are Labour members but have not been obliged to take the whip.

The MP, who lost a confidence vote within his local party over his support for Brexit, said he would not trigger a byelection.

The Labour leader, Jeremy Corbyn, expressed bafflement on Monday at Field’s decision to resign. “Frank has been a Labour MP for a very long time, even longer than I have. I’ve known him for a very long time. I’ve worked with him in the past on social security and other issues,” he said in Liverpool.

“I’m sorry he’s resigned and I thank him for all the work that he’s done as an MP and for the party, but I don’t see why he had to resign.”

A party spokesperson said Labour did not comment on personal matters.