Labour MP Chris Williamson has said "I deeply regret, and apologise for, my recent choice of words" after The Yorkshire Post revealed his claim that the party had "given too much ground" in its response to complaints of anti-Semitism.
The apology came after a party spokesman said that the Derby North MP - a close ally of leader Jeremy Corbyn - should withdraw his "deeply offensive and inappropriate" remarks, which fell below the standards expected from MPs.
Video footage showed Mr Williamson telling a meeting of the grassroots Momentum group that Labour's reaction to anti-Semitism allegations had been "too apologetic" and had led to the party being "demonised".
He had already provoked the party's ire by booking a room in Parliament for the screening of a film about an activist suspended for alleged anti-Semitism, something which a Labour spokesman described as "completely inappropriate".
The video, obtained exclusively by The Yorkshire Post, was recorded at a meeting in Sheffield in the wake of last week's resignation of eight Labour MPs to join the Independent Group.
It showed Mr Williamson receiving loud applause as he complained that the party had "given too much ground" to its critics.
He was also filmed saying he had celebrated the resignation of MP Joan Ryan, who quit Labour in protest over the handling of anti-Semitism and bullying complaints.
In a "personal message and sincere apology from me regarding my recent remarks on anti-Semitism in the Labour Party" posted on Twitter, Mr Williamson said: "The Labour Party is an anti-racist party. It is the only party that has stood shoulder-to-shoulder with religious and ethnic minorities in their decades-long fight against racism, discrimination and prejudice in the United Kingdom.
"On a personal level, I have been an anti-racist all my life. As a former member of the Anti-Nazi League, I participated in direct action to confront foul anti-Semites in the streets. I reject racism ethically and morally. It has no place in the Labour Party or in our country.
"It pains me greatly, therefore, that anyone should believe that it is my intention to minimise the cancerous and pernicious nature of anti-Semitism. I deeply regret, and apologise for, my recent choice of words when speaking about how the Labour Party has responded to the ongoing fight against anti-Semitism inside of our party. I was trying to stress how much the party has done to tackle anti-Semitism.
"Our movement can never be "too apologetic" about racism within our ranks. Whilst it is true that there have been very few cases of anti-Semitism in the Labour Party - something I believe is often forgotten when discussing this issue - it is also true that those few are too many.
"It is precisely because of our party's historic struggle against racism that we have taken it upon ourselves to strengthen our rules, to improve our disciplinary procedures and to redouble our efforts to take on anti-Semites. We have held ourselves to a higher standard than any other political party when it comes to anti-racism - and rightly so.
"I am therefore sorry for how I chose to express myself on this issue within our party. This is a fight that I want to be an ally in. In future, I will take it upon myself to be more considered in my remarks, and ensure they reflect the Labour Party's unswerving and unfaltering commitment to anti-racism and the fight against anti-Semitism."
A Labour spokesman said: "These comments are deeply offensive and inappropriate and fall below the standards we expect of MPs.
"Downplaying the problem of anti-Semitism makes it harder for us to tackle it. Chris Williamson should apologise immediately and withdraw his remarks."
Responding to the video on Twitter, Jewish MP Luciana Berger - who also quit Labour for the Independent Group - said: "This is what I have left behind. It's toxic. Our country deserves so much better. #ChangePolitics".
And the comments were condemned by sitting Labour MPs, including deputy leader Tom Watson, who described Mr Williamson's behaviour as "deliberately inflammatory".
Mr Watson said he reported the matter to the chief whip and general secretary "as soon as" he found out about it.
Hull MP Diana Johnson tweeted: Labour MPs this morning are standing up and calling for him to have the whip withdrawn. It’s absolutely unacceptable that he remains.
Meanwhile, Cardiff South MP Stephen Doughty tweeted: "This conduct is unacceptable and has no place in our party. I have made clear to both the leader's office and our whips that I expect urgent action to be taken."
Birmingham Yardley MP Jess Phillips said Mr Williamson's comment seemed "specifically designed to upset", adding: "Must be disciplined."
Walthamstow MP Stella Creasy said anti-Semitism has to be "expunged not ignored", tweeting: "Those who perpetuate it or minimise it bring the party into disrepute. No ifs, no buts. #standupspeakup".
And Ilford North MP Wes Streeting tweeted: "Stomach-turning. No action will be taken."
In the video, Mr Williamson said: "The party that has done more to stand up to racism is now being demonised as a racist, bigoted party.
"I have got to say I think our party's response has been partly responsible for that because in my opinion ... we have backed off on too much, we have given too much ground, we have been too apologetic."
Mr Williamson also said he had sung Celebration by Kool & The Gang after hearing of Ms Ryan's resignation.
During a visit to Derbyshire in January, Mr Corbyn said Mr Williamson was a "very good, very effective Labour MP".
The Labour leader told Derbyshire Live: "He's a very strong anti-racist campaigner. He is not anti-Semitic in any way."
Mr Williamson was first elected MP for Derby North in 2010, but was unseated in the 2015 general election.
He ran again in 2017, this time receiving a visit from Mr Corbyn during the general election campaign, and won.
Mr Williamson was later made a shadow fire and emergency services minister.
He left the post by mutual agreement six months later after commenting on policy outside his brief to suggest that council tax should be doubled for better-off homes.