Unite leader Len McCluskey has warned Labour MPs who attack Jeremy Corbyn over anti-Semitism that they "can expect to be held to account".
Mr McCluskey – one of the Labour leader's key allies – backed action to stamp out anti-Semitism.
But he accused "Corbyn-hater MPs" of using the issue to try to "toxify the Labour Party".
Labour frontbencher Sir Keir Starmer told BBC Radio 4's Today programme: "I am afraid I disagree with Len on this."
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Labour has been dogged by a series of incidents including criticism last month of Mr Corbyn's comments on an allegedly anti-Semitic mural in 2012 and the party's suspension of former London Mayor Ken Livingstone over his claim that Adolf Hitler had supported Zionism in the 1930s.
Mr Corbyn has apologised for anti-Semitism – prejudice against Jewish people – in his party and said he was "absolutely committed" to rooting it out.
But several Labour MPs have criticised his response, and in a Commons debate last week the leadership was urged to do more to combat the problem within the party.
Writing in the New Statesman, Mr McCluskey – who has previously dismissed the anti-Semitism row as "mood music" – said anti-Semites "have no place in the party", called for the party's Chakrabarti report to be implemented in full, and for the case of former London mayor Ken Livingstone – who has been suspended since 2016 – to be "resolved".
But he criticised "a few dozen MPs" who he said "appear to wake up each morning thinking only: 'How can I undermine Jeremy Corbyn today?'"
Naming MPs including Chris Leslie and Wes Streeting, he accused them of a "sustained smearing" of the leader.
Mr McCluskey said he did not personally support mandatory re-selection – meaning all MPs would face an open selection battle before an election – but added: "I look with disgust at the behaviour of the Corbyn-hater MPs who join forces with the most reactionary elements of the media establishment and I understand why there is a growing demand for mandatory reselection."
He added: "To watch as these so-called social democrats tried to demean and attack, in front of our enemy, a decent and honourable man who has fought racism and anti-Semitism all his life and who has breathed life and hope back into the hearts of millions, especially the young, made my stomach churn. To see Tory MPs cheer and applaud them was shameful.
"Promiscuous critics must expect to be criticised, and those who wish to hold Corbyn to account can expect to be held to account themselves."
Shadow Brexit Secretary Sir Keir Starmer said he disagreed with the Unite leader.
"Jeremy Corbyn has made it clear, and it is obvious, that we have got a problem with anti-Semitism. We have got to deal with it robustly and effectively," he told BBC Radio 4's Today programme.
"Part of that is the disciplinary procedure, which needs to be much quicker and much more effective, but there is also a cultural question.
"Part of that cultural question is to stop those denying that there is even a problem. That is part of the problem. So I am afraid I disagree with Len on this."
One of the MPs singled out by Mr McCluskey, Neil Coyle, accused the union chief of undermining Mr Corbyn's pledge to tackle anti-Semitism. Mr Streeting said he would not be deterred from voicing Jewish constituents' concerns, and Mr Woodcock told Mr McCluskey to focus on protecting Unite members' jobs.
Skip Twitter post by @wesstreeting
I’ve said it before and I’ll say it again: no abuse, intimidation or threats of deselection will prevent me from voicing the concerns of my Jewish constituents about antisemitism in the Labour Party.
— Wes Streeting MP (@wesstreeting) April 25, 2018
End of Twitter post by @wesstreeting
Mr McCluskey's intervention comes after Mr Corbyn held what he described as "positive and constructive" talks on Tuesday with the Jewish Leadership Council and the Board of Deputies of British Jews.
But the groups' leaders took a different view, saying the talks were a "disappointing, missed opportunity" and did not agree a minimum plan of action.
By Iain Watson, BBC political correspondent
Accusations that anti-Semitism wasn't being taken seriously enough by Labour are proving politically toxic in two ways.
First, they undermine Labour's "brand" – a party that stands for equal rights and anti-racism.
And secondly, the effort to detoxify the issue has arguably diverted the party's leadership from the forthcoming local election contests.
So it would have been helpful if the Jewish groups had suggested that progress had been made since their demonstration in Parliament Square, which some of Jeremy Corbyn's own MPs joined a month ago.
While they did welcome Mr Corbyn's words on anti-Semitism – and his denunciation of those who said the problem had been invented or exaggerated – their verdict on action, or lack of it, means that the issue remains a potent force in the hands of the Labour leader's internal and external opponents.
Perhaps the most positive interpretation is that there was a disagreement over detail and not a fundamental division in outlook.
That's certainly how sources close to the Labour leadership would like it to be seen.
- Read more from Iain