Len McCluskey has been forced to apologise after he was accused of levelling an anti-Semitic trope at Labour grandee Lord Mandelson.
Mr McCluskey, the Unite general secretary, was on Tuesday facing a widespread backlash after telling the Blair-era minister, who is of Jewish descent, to leave politics and "count his gold".
His remarks came after he was asked about Lord Mandelson's praise for the new Labour leader, Sir Keir Starmer.
Mr McCluskey, a prominent ally of Jeremy Corbyn, told BBC Newsnight: "I would suggest that Peter just goes into a room and counts his gold and not worry about what's happening in the Labour Party. Leave that to those of us that are interested in ordinary working people."
His comments provoked an outcry among leading Jewish and Labour figures including the veteran MP Dame Margaret Hodge, who said: "Why McCluskey uses tropes that many would consider anti-Semitic on BBC Newsnight is a question only he can answer.
"Regardless, he doesn't get to obfuscate and dictate to us what is and is not anti-Semitic when called out."
Lord Adonis, a former transport secretary, said: "Len McCluskey's vile personal remarks on Newsnight just now sum up the moral as much as the political bankruptcy of the Corbyn far-Left project."
Mike Katz, the national chairman of the Jewish Labour Movement, said he was "pretty disgusted" by the remarks.
The fallout prompted a swift apology from Mr McCluskey, who wrote on Twitter: "Before this gets out of hand, let me say language is important and I apologise to Peter Mandelson and anyone else if mine has caused hurt."
However, in a move that fuelled further outcry, a statement issued by Unite denied that Mr McCluskey had intended to highlight Lord Mandelson's Jewish heritage.
"Mr Mandelson's religion was not relevant to the comments made by Mr McCluskey," it said. "Indeed, to the best of our knowledge Mr Mandelson is not Jewish.
"The ordinary meaning of the statement made by Mr McCluskey is one of his beliefs that in recent years Mr Mandelson has had more interest in increasing his own wealth than in fighting for social justice for working class people.
"The suggestion of any anti-Semitic meaning to the commentary would be ludicrous."