Unite chief Len McCluskey has said the row about antisemitism is being used to undermine Jeremy Corbyn.
Mr McCluskey’s comments, came as the months-long row over antisemitism allegations in the party reached new heights in recent days.
Ex-mayor Ken Livingstone and MP Naz Shah were both suspended from the party for comments they made.
The union boss attacked the accusations as "cynical attempt to manipulate antisemitism for political aims".
Mr McCluskey whose union is Labour's biggest donor, told the BBC that the fight had been started by party grandees who "get out of their wheelchair and toss a few hand grenades in".
Another defender of Mr Corbyn, MP Diane Abbott, said it was "a smear to say that the Labour Party has a problem with anti-Semitism".
Speaking on the BBC One Andrew Marr Show, she said Mr Livingstone's comments linking Hitler with Zionism - were "extremely offensive" but not part of any wider problem specific to her party.
She said she would be "dismayed if some people were hurling around accusations of antisemitism as part of some intra-Labour Party dispute".
Meanwhile, Jewish donors are reported to have withdrawn support for the party, including Sir Ronald Cohen.
Mr Cohen a social investment pioneer said: "There is no room for racism among the values of the Labour party.
“If the leadership does not stamp out racism now, racism will stamp out the Labour party."
He donated more than £2.5m to Labour under Tony Blair and Gordon Brown.
Reports suggest Labour frontbenchers are threatening to resign within weeks over Mr Corbyn's handling of the party's antisemitism row.
According to The Times one MP said that a meeting of the party's MPs, which is due to take place next week, is expected to be "absolute carnage".
A source told The Times: "People are very, very angry - livid - not only about getting in the mess in the first place, but then digging frantically.
"When we get beyond Thursday, I think people may well be a bit more vocal.
"There may also be one or two people who want to jack it in and call it a day."