Veteran MP Frank Field has said he is considering triggering a by-election in his Birkenhead constituency.
He has quit the Labour whip over the leadership’s handling of anti-Semitism allegations but wants to continue sitting as an “independent Labour” MP.
He also attacked the “culture of nastiness” in Labour under Jeremy Corbyn’s leadership.
Mr Corbyn’s supporters said he had quit because he was facing de-selection in his Birkenhead constituency.
He recently lost a vote of confidence organised by local party members angry at his support for the government in Brexit votes.
Mr Field, who has represented Birkenhead since 1979 and has faced previous efforts to de-select him, denied he had “jumped before he was pushed” and said he intended to fight the next general election as an independent Labour candidate.
He told BBC Breakfast he would spend the “next few days” deciding whether to stand down as an MP to trigger a by-election, in which he would stand against an official Labour candidate.
“I will obviously make a decision about whether I should actually have a by-election or not… I will be in Birkenhead, people will be talking to me, coming up to me in the street to see whether they want me to have a by-election or not”.
Mr Field quit the party’s parliamentary group through a letter to chief whip Nick Brown on Thursday, saying the leadership was becoming “a force for anti-Semitism in British politics”.
A Labour Party spokesman has thanked Mr Field for his service to Labour, but the veteran MP told the BBC he had been thanked “as if I was resigning from a whist club”, noting he had been in the party longer than leader Jeremy Corbyn.
The MP also blamed a “culture of intolerance, nastiness and intimidation” in local Labour parties.
A Labour source said “Frank has been looking for an excuse to resign for some time” and, under party rules, he could not resign as a Labour MP and continue to be a member of the party.
Labour’s deputy leader Tom Watson described the resignation of Mr Field as a “serious loss to the party” and a “major wake up call”.
And fellow MP Siobhain McDonagh has described Frank Field as “a Labour warrior”, telling BBC’s Newsnight: “Don’t we want in our politicians and our MPs mavericks who are prepared to stand up and say it as it is?”
He was also applauded by some Conservative MPs, with Home Secretary Sajid Javid tweeting that he was “a man of integrity and principle”.
But Corbyn-supporting members of Labour have criticised Mr Field’s decision.
“He’s obviously lost the confidence of his members and he’s now getting his excuses in, it seems to me, and throwing around grotesque slurs, which have no basis in reality,” said MP Chris Williamson.
“The party has taken the issue of anti-Semitism very seriously; far more seriously than any other political party.
“It’s so sad to see someone like Frank Field trashing Labour’s anti-racist record”.
And Labour’s shadow justice secretary Richard Burgon said Mr Field should face a by-election.
Labour has been dealing with a row about the extent of anti-Semitism within the party for more than two years.
A 2016 inquiry, carried out by Shami Chakrabarti, concluded that while the Labour Party was not overrun by anti-Semitism, there was an “occasionally toxic atmosphere”.
Recently, the focus has been on a new code of conduct the party has adopted on anti-Semitism, with critics concerned that it does not go as far as the International Holocaust Remembrance Alliance’s guidelines.
Earlier this month, Mr Corbyn again apologised for hurt caused to Jewish people by anti-Semitism in his party and admitted Labour had been too slow in dealing with disciplinary cases.