Donald Trump’s former campaign chairman was working with a former Russian spy during the US presidential race over debts he owed to a Kremlin-linked oligarch, it has been claimed.

Paul Manafort led Mr Trump’s campaign for president for three months in 2016 before he was forced to resign after his links to Russian interests in Ukraine were made public.

Now a former member of the GRU, Russia’s most feared and secretive spy service, has said he was in contact with Manafort during that time over millions of dollars the political adviser allegedly owed to billionaire businessman Oleg Deripaska.

“He owed us a lot of money, and he was offering ways to pay if back,” Victor Boyarkin told Time, in reference to Manafort in 2016.

Mr Boyarkin also told the magazine he had been approached by special counsel Robert Mueller’s office, which is probing possible collusion between Russia and the Trump campaign, but that he told investigators “to go dig a ditch”.

His claims could be relevant to Mr Mueller’s investigation as they would present some of the clearest evidence of leverage powerful Russians had over a senior member of Mr Trump’s team.

Mr Boyarkin was last month placed at the top of the US government’s latest sanctions list against high-profile Russians involved in Moscow’s “continued regard for international norms”.

The Treasury Department document described Mr Boyarkin as a former GRU officer who “reports directly to Deripaska and has led business negotiations on Deripaska’s behalf”.

In a 2015 complaint filed to a court in Virginia, Mr Deripaska, a metals magnate, alleged Manafort owed him $19m (£14.95m) in relation to a failed business investment in Ukraine.

Former Trump campaign chairman Paul Manafort pleads guilty

Lawyers for Mr Deripaska complained in a petition filed in the Cayman Islands a year earlier that Manafort had “simply disappeared”.

Mr Boyarkin said it was left to him to reclaim the money on Mr Deripaska’s behalf. “I came down on him hard,” he said.

When Manafort reappeared, serving initially in April 2016 as an unpaid adviser to the Trump campaign, the 69-year-old tried to offer Mr Deripaska “private briefings” about the election, apparently in an attempt to “get whole”, according to emails seen last year by The Atlantic.

He attempted to do this through an old associate, Konstantin Kilimnik, who had worked for Manafort for a decade in Ukraine.

There is no evidence any meetings occurred between Manafort and Mr Deripaska, and it is unknown if Manafort is actually indebted to the oligarch, or if he paid him back.

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Mr Boyarkin’s assertion he was in contact with Manafort, in his role as debt collector for Mr Deripaska, would appear to contradict the latter’s denials last year of any contact with Manafort during the presidential election.

Mr Deripaska, one of Russia’s wealthiest men, remains under US sanctions imposed in April in retaliation for alleged Russian meddling in the 2016 presidential election.

He amassed his fortune under Mr Putin and has bought assets abroad in ways widely perceived to benefit the Kremlin’s interests.

US diplomatic cables from 2006 described him as “among the two or three oligarchs Putin turns to on a regular basis” and “a more-or-less permanent fixture on Putin’s trips abroad”.

The 50-year-old achieved a deal of fame in the UK in 2008 when the then-business secretary, Lord Mandelson, and shadow chancellor George Osborne found themselves aboard his yacht off the coast of Corfu last summer.

Mr Deripaska has been contacted for comment.

Manafort was convicted of eight financial crimes in August, uncovered by Mr Mueller’s team, and faces sentencing early next year.