Former minister Ian Austin has quit Labour, saying he is "ashamed" of the party under Jeremy Corbyn's leadership and "appalled" by the offence it has caused to Jews.
He said there was a "culture of extremism, anti-Semitism and intolerance" as he became the ninth MP to quit Labour in a week.
The Dudley North MP, whose Jewish adoptive father was forced to flee the Nazis as a child, said he joined Labour to fight racism and "I could never have believed that I'd be leaving because of racism too".
I have decided to leave the Labour Party and I wanted to tell people in Dudley first. https://t.co/IaXSwDry06
— Ian Austin (@IanAustinMP) February 22, 2019
He said he could never ask his constituents to make Mr Corbyn prime minister, claiming the Labour leader and shadow chancellor John McDonnell "cannot be trusted with our national security and would undermine our democratic institutions".
Mr Austin said he had no plans to join his eight former colleagues in the new Independent Group they had set up this week.
Labour called for Mr Austin to quit as an MP and fight a by-election in the seat he held in 2017 with a majority of just 22.
A party spokesman said: "We regret that Ian Austin has left the Labour Party.
"He was elected as a Labour MP and so the democratic thing is to resign his seat and let the people of Dudley decide who should represent them."
But deputy Labour leader Tom Watson said it was a "serious blow" to the party and added: "It's also personally hard to see a close friend take a decision of this magnitude."
Very sad to lose another colleague from the Labour team. It's also personally hard to see a close friend take a decision of this magnitude. https://t.co/Q2whZXcuJx
— Tom Watson (@tom_watson) February 22, 2019
Mr Austin announced his decision in the Express & Star newspaper and issued a fuller explanation in a statement on his website.
"I grew up listening to my Dad – a refugee from the Holocaust – teaching me about the evils of hatred and prejudice," he said.
"One of the main reasons I joined the Labour Party as a teenager here in Dudley more than 35 years ago was to fight racism, and I could never have believed that I'd be leaving because of racism too."
He added: "Jeremy Corbyn and the people around him have turned a mainstream party into something very different.
"He has spent his entire political career working with and supporting all sorts of extremists, and in some cases terrorists and anti-Semites.
"I always thought he was unfit to lead the Labour Party and I certainly think he is unfit to lead our country."
Former Labour MPs Luciana Berger, Chuka Umunna and Chris Leslie – who left to form the Independent Group earlier in the week – offered support to Mr Austin after his announcement, although he has said he has no plans to join them.
Massive respect to @IanAustinMP for making this incredibly difficult decision – it's painful and hard but he has stayed true to his values and what he believes to be the national interest. https://t.co/TAEnoCJwFK
— Chuka Umunna (@ChukaUmunna) February 22, 2019
Full respect for @IanAustinMP and his difficult decision to leave @UKLabour today. Politics is broken and it is so important MPs stand up for the mainstream values we share. #ChangePoliticshttps://t.co/uOiLerQYkl
— Chris Leslie (@ChrisLeslieMP) February 22, 2019
— Luciana Berger (@lucianaberger) February 22, 2019
Khalid Mahmood, Labour MP for Birmingham Perry Barr, called Mr Austin's decision to quit the party "very sad".
He told the Press Association: "I've known Ian a long time, before we were MPs and he has been a good friend. It's regrettable that he's done this."
He added that Labour under Mr Corbyn was "doing its best" on anti-Semitism, and was operating a "zero-tolerance" policy on the issue.
He said: "Unfortunately there are those associated with the party who make comments.
"But the party is doing its best and will continue to have zero tolerance against anti-Semitism."