Veteran MP Frank Field has resigned the Labour whip, describing the party as a "force for anti-Semitism".
Field, who has represented Birkenhead for almost 40 years, said in a letter to party chief whip Nick Brown that he could no longer serve the party as the leadership was overseeing an "erosion of our core values".
The one-time minister for welfare reform, who was charged by Tony Blair to reform the welfare state by "thinking the unthinkable", also rounded on his local party.
In the one-page letter he said the Birkenhead Constituency Labour Party was now governed by "a culture of intolerance, nastiness and intimidation".
'Increasingly seen as a racist party'
Field has faced criticism from party members for his voting record on key Government Brexit legislation.
Earlier this year he was one of four Labour MPs who voted with the Government to prevent Britain joining a post-EU customs union, on the grounds that working-class voters "gave politicians a clear instruction to take the country out of the EU".
In his letter Mr Field said: "I am resigning the whip for two principal reasons.
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"The first centres on the latest example of Labour's leadership becoming a force for antisemitism in British politics.
"The latest example, from last week, comes after a series of attempts by Jeremy [Corbyn] to deny the 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 also raised concern about the "thuggish conduct" of some members in his local party and the lack of action against them.
He said the party's actions served to "legitimise appalling levels of bullying and intimidation".
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Field said that he intended to continue representing Birkenhead in Westminster as an Independent.
He added: "I also intend, providence willing, to represent those views when the next election is called."
Chairman: Resignation is 'damning'
Responding, Brandon Lewis MP, Conservative Party Chairman, said: “Frank Field’s resignation is a damning indictment of Jeremy Corbyn’s total inability to take action against bullying and anti-Semitic racism within Labour.
“Despite promising a ‘kinder politics’, time and time again Corbyn has allowed people to get away with doing and saying things which have no place in public life.
“If other Labour MPs don’t follow Frank Field’s example, they are continuing to endorse Jeremy Corbyn’s abject failure to act.”