Frank Field has dramatically resigned from the Labour Party whip, accusing Jeremy Corbyn of becoming a “force for anti-Semitism”.
The MP for Birkenhead, who has held the seat since 1979, said the party was now in the grip of a “culture of intolerance, nastiness and intimidation”.
Field, who currently serves as chairman of the Commons Work and Pensions Committee, will continue to sit in parliament as an independent.
Corbyn has spent the summer denying he either holds anti-Semitic views, or that he tolerates them among his supporters.
A Labour source said: “Frank has been looking for an excuse to resign for some time.”
Conservative Party Chairman Brandon Lewis MP said Field’s resignation was “a damning indictment of Jeremy Corbyn’s total inability to take action against bullying and anti-Semitic racism within Labour”.
In his letter to Labour Chief Whip Nick Brown, Field said the party was guilty of tolerating “thuggish” behaviour by some of its members.
“I am resigning the whip for two principal reasons. The first centres on the latest example of Labour’s leadership becoming a force for anti-Semitism in British politics,” he said.
“The latest example, from last week, comes after a series of attempts by Jeremy to deny that past statements and actions by him were anti-Semitic.
“Britain fought the Second World War to banish these views from our politics, but that superhuman effort and success is now under huge and sustained internal attack.
“The leadership is doing nothing substantive to address this erosion of our core values. It saddens me to say that we are increasingly seen as a racist party. This issue alone compels me to resign the whip.”
Field said “a culture of intolerance, nastiness, and intimidation now reigns in too many parts of the party nationally and is sadly manifest within my own Constituency Labour Party (CLP) in Birkenhead”.
“This is, I fear, just one example of a phenomenon that has tightened its grip on CLPs across the country and is being driven, in part, by members who In previous years would never have been able to claim Labour Party membership,” he said.
The veteran MP also referred in his letter to a dispute with his local party – in which he lost a no confidence vote last month after voting with the government on Brexit.
Deputy Labour leader Tom Watson said he “deeply regretted” Field’s decision.
“This is a serious loss to the party,” he added in a statement.
“It reflects both the deep divisions in the party and the sense of drift engulfing us. It is a major wake up call. We cannot afford to lose people of such weight and stature”.
Home Secretary Sajid Javid tweeted about Field’s resignation, adding that it was “sad what Labour has become”.
He said: “First met Frank Field when I was 19 and he spoke at the University of Exeter. Admired him ever since.
“We have our political differences, but he’s a man of integrity and principle. Sad what UK Labour has become.”
Labour MP Wes Streeting warned that if the issues raised by Field are not addressed, the party could see “a full-blown existential crisis”.
According to party rules, it is not possible to resign as a Labour MP and remain a member of the party and Field is expected to have talks with the whips’ office on Friday.
Labour’s shadow justice secretary Richard Burgon said Field should call a by-election.
In a tweet, he said: “Politicians who are elected as Labour MPs by their constituents and who then leave the Labour Party should do the right and respectful thing and call a by-election straight away.”
Next week Labour’s ruling National Executive Committee (NEC) will again considers its code of conduct on anti-Semitism.
The party’s initial decision not to adopt the full International Holocaust Remembrance Alliance (IHRA) definition of anti-Semitism triggered a backlash from some MPs worried it would allow anti-Jewish hatred to flourish within the party.
Earlier this week a senior party official suggested Field would “walk” an election in his seat if he chose to stand as an independent.
Sheila Murphy, who ran Yvette Cooper’s Labour leadership campaign, claimed Field’s local party is in the grip of “nasty, vicious, and controlling” activists who support Corbyn.
In an interview with Labour Uncut, Murphy suggested Field, 76, who has been MP for the Wirral constituency since 1979, was under threat of deselection from “out of control” new members.
Yesterday a Merseyside Labour councillor quit the party and said his local party had been “over-run by a narrow, ideological cult”.
Mike Sullivan said “parasites” and people with “extremist” views now held positions of influence.
Field has a majority of 25,514. In July his local party passed a no confidence motion against him after he sided with Theresa May in Brexit votes.