Andrew Murray, chief of staff for Unite general secretary Len McCluskey, was a member of the far-left organisation as recently as December.
Mr Murray’s previous comments are already coming back to haunt him – he once declared “solidarity” with the North Korean regime against the US.
Formerly a journalist at the left-wing Morning Star newspaper – a favourite read of Mr Corbyn – Mr Murray is also an ex-chair of the Stop the War Coalition, which led the protests against the war in Iraq.
In fact Mr Corbyn succeeded him as chairman of the organisation after he stepped down in 2011.
He became a fully paid-up member of the Communist Party of Great Britain in 1976 and previously said: “Communism still represents, in my view, a society worth working towards – albeit not by the methods of the 20th century, which failed.”
His stance on Joseph Stalin – who regime was responsible for the death and suffering of millions of people – is well documented.
In 1999 he wrote “If you believe that the worst crimes visited on humanity this century, from colonialism to Hiroshima and from concentration camps to mass poverty and unemployment have been caused by imperialism, then [Stalin’s birthday] might at least be a moment to ponder why the authors of those crimes and their hack propagandists abominate the name of Stalin beyond all others.”
In 2003 Mr Murray expressed his “solidarity” towards North Korea for resisting efforts by the US to enact a regime change.
He told an executive committee meeting of the Communist Party of Britain: “Our Party has already made its basic position of solidarity with Peoples Korea clear.”
More recently Mr Murray, who joined the Labour Party in November 2016 following Mr Corbyn’s second election as leader, downplayed the Paris terror attacks, appearing to shift attention to other “barbarism”.
After being introduced by Mr Corbyn, he told a panel: “The barbarism we condemn in Paris is minute compared to the barbarism wrought by imperialism across the planet in the last 13 years and we must condemn that.
“It is a sad lesson we have to re-learn from the attacks in Paris, it needs bringing home again and again.”
Sources in Mr Corbyn’s camp defended the move to bring in Mr Murray, describing him as a “grown up” who has extensive experience managing Unite, and will be able to provide “support” to Labour’s General Election campaign.
Recent polls have seen a slight upsurge in support for Labour, with some putting them on 32% – although the Tories are still polling in the high 40s, putting them into potential landslide territory.
Labour insiders have suggested that Mr Corbyn will stay on as leader even if the party is trounced in the June vote.
The Communist Party of Great Britain are not fielding any candidates for the election this year and have urged people to vote for Labour to get a “left-led government”.
Top pic: Rex