The investigation is believed to centre around a series of antisemitic Twitter messages sent last weekend to Mr Coyne from an account set up in recent months under the pseudonym of @1UnionBloke.
Among of series of anti-Jewish messages sent from the account was one featuring a superimposed photo of Mr Coyne’s face on an image of a Jewish man brandishing a gun, with the wording: “Be warned @gerard_coyne has backing now!!”, along with the phrase “Jewish Mafia”.
Other messages from the account, all of which attempted to promote support for Mr McCluskey’s campaign, included suggestions Mr Coyne was pandering to the Jewish community in his leadership campaign.
One tweet stated: “His anti anybody who isn’t Jewish is exposed. @gerard_coyne also reveals his shadowy backers. So @UniteforLen.”
Another Twitter post from a different account claimed Mr Coyne was “cosying up to powerful Jewish conservative supporters of Zionism and ready to sell his soul”.
Unite union sources said there are concerns the individual operating the @1UnionBloke account also had “access to confidential local branch matters only known to a small clique at the top of the union”.
The spate of messages sent to Mr Coyne from social media accounts received widespread condemnation.
Observer journalist and author Nick Cohen wrote: “Len McCluskey’s supporters are disgusting. There’s no racist propaganda too disgusting for them to use against Gerard Coyne.”
In an apparent attempt to limit any investigation into the @1UnionBloke account, it was closed down on Sunday morning with a statement issued to Mr Coyne stating: “Following advice I unreservedly apologise for any offence caused to you by recent tweets and will be de-activating this account.”
The antisemitic smear campaign was apparently sparked by Mr Coyne’s attack on Mr McCluskey’s leadership of Unite. The candidate claimed the incumbent had repeatedly “singled out” Israel for condemnation.
A spokeswoman for Mr McCluskey’s campaign said: “Len has always been clear that he wants his supporters to support his positive, progressive agenda, which celebrates the inclusive values of our labour movement.
“He does not want or condone any engagement by them in online abuse.
“Unite has also made it clear that any member who has made antisemitic comments is in breach of the union’s rules and will face disciplinary action.”
The forthcoming Unite leadership election has become the battleground for the bitter civil war that has erupted between the far-left and more moderate forces within the Labour Party.
Mr McCluskey is a strong supporter of both Mr Corbyn and far-left Momentum founder Jon Lansman, while Mr Coyne is close to Labour’s deputy Tom Watson.
Ballot papers in the Unite election go out on March 27 — with the result due at the end of April
