Jeremy Corbyn has been dealt a blow as veteran MP Dame Louise Ellman quit Labour accusing him of being a danger to Britain.
Dame Louise, 73, who is Jewish, said she had been "deeply troubled" by the "growth of anti-Semitism" in Labour in recent years.
She told ITV News Political Correspondent Daniel Hewitt she believes the party is now "institutionally anti-Semitic" and claimed Jeremy Corbyn has "presided over" the growth of anti-Semitism in the Labour Party.
Mr Corbyn, responding to the resignation, thanked Ms Ellman for her work but rejected her criticisms of him and the Labour Party.
"We do not tolerate anti-Semitism in any form whatsoever...and I will hold that position to my dying day," he said.
She said she had not yet decided whether to run at the next election as an independent, but said she would "never join another political party".
Ms Ellman said she hopes to return to her "political home" in the future, if Mr Corbyn steps down or is removed as leader.
She tweeted: "I have made the truly agonising decision to leave the Labour Party after 55 years.
"I can no longer advocate voting Labour when it risks Corbyn becoming PM."
The Liverpool Riverside MP added: "I believe that Jeremy Corbyn is not fit to serve as our Prime Minister. With a looming general election and the possibility of him becoming Prime Minister, I feel I have to take a stand."
And she told The Times newspaper that if he became prime minister "I believe that Jeremy Corbyn would be a danger to the country, a danger to the Jewish community as well, but a danger to the country too".
In a statement, Dame Louise said she felt Labour was not a safe place for Jewish people.
She said: "Under Jeremy Corbyn's leadership, anti-Semitism has become mainstream in the Labour Party. Jewish members have been bullied, abused and driven out.
"The Labour Party is no longer a safe place for Jews and Jeremy Corbyn must bear the responsibility for this.
"We cannot allow him to do to the country what he has done to the Labour Party.
"The overwhelming majority of the Jewish community is fearful of what a Corbyn government might mean for Britain's Jews. I share those concerns.
"But this issue is not simply about the Jewish community. This is about the nature of our society.
"Jeremy Corbyn's seeming tolerance of anti-Semitism would embolden racists, poison our public debate and damage the social cohesion of our country.
"My values - traditional Labour values - have remained the same. It is Labour, under Jeremy Corbyn, that has changed.
"He has presided over a culture of hatred, fear and intolerance in the Labour Party.
"But this issue is no longer just about the Labour Party - it is about the threat a Jeremy Corbyn premiership could pose to the country."
Dame Louise said she would not join another political party and hoped to return to Labour under a different leadership.
The MP had been due this week to face so-called "trigger" ballots which had the potential to lead to her deselection as a candidate at the next election.
A Labour Party spokesperson said: "Jeremy Corbyn thanks Louise Ellman for her service to the Labour Party over many years.
"Jeremy Corbyn and the Labour Party are fully committed to the support, defence and celebration of the Jewish community and continue to take robust action to root out anti-Semitism in the party and wider society.
"Jeremy Corbyn has consistently supported struggles for human rights and justice around the world and made the right calls in the interests of security and peace."
Labour former deputy leader Harriet Harman described the resignation of Dame Louise as "very sad news".
She tweeted: "Thank you, Louise, for your terrific contribution to Parliament, politics and the Labour Party!"
Liberal Democrat former leader Tim Farron tweeted: "I've known Louise for 30 years.
"If there's no place for Louise Ellman in the 2019 Labour Party, they are finished as a serious force."
Appearing on ITV's Peston, Labour MP Stephen Kinnock called Ms Ellman's resignation a "tragedy" and a "massive loss to our movement".
He added the 73-year-old was "one of the most widely respected and liked across the Parliamentary Labour Party" and her decision to quit was due to the effects of "anti-Semiticism rearing it's head".
Also speaking to ITV News Political Editor Robert Peston was Labour MP David Lammy, said Ms Ellman's resignation left him "very sad".
Mr Lammy said Ms Ellman "was a beloved member of the Parliamentary Labour Party" and someone who was "very senior and very respected" within in.