Donald Trump’s former personal lawyer says he is providing “critical information” in Robert Mueller’s probe into Russian interference in the 2016 US election and possible co-ordination between Moscow and the Trump campaign.

Michael Cohen, who pleaded guilty to campaign finance and other charges last month, said he is providing the information to prosecutors without a co-operation agreement.

The president’s longtime fixer-turned-foe could be a vital witness for prosecutors as they investigate whether the Trump campaign co-ordinated with Russians.

For more than a decade, Cohen was Mr Trump’s personal lawyer, a key player in the Trump Organisation and a fixture in the president’s political life.

Donald Trump
Donald Trump has said he will declassify secret documents in the Russia investigation (Evan Vucci/AP)

Cohen pleaded guilty in August to eight federal charges and said Mr Trump directed him to arrange payments before the 2016 election to buy the silence of porn actress Stormy Daniels and a former Playboy model who both alleged they had affairs with Mr Trump.

It was the first time any Trump associate implicated the president himself in a crime, though whether a president can be prosecuted remains a matter of legal dispute.

On Thursday night, Cohen tweeted: “Good for @MichaelCohen212 for providing critical information to the #MuellerInvestigation without a cooperation agreement. No one should question his integrity, veracity or loyalty to his family and country over @POTUS @realDonaldTrump.”

The tweet was deleted almost immediately, and later reposted by his lawyer, Lanny Davis, who said he wrote the tweet for Cohen and asked him to tweet it because he has a “much larger following”.

Mr Davis said he was delayed posting the tweet on his own account, so Cohen tweeted it first.

ABC News reported earlier that Cohen has several times met investigators from special counsel Mr Mueller’s office.

The television network said he was questioned about Trump’s dealings with Moscow, including whether members of his campaign worked with Russians to try to influence the outcome of the election.

Mr Davis said last month that his client could tell the special counsel that Mr Trump had prior knowledge of a June 2016 meeting at Trump Tower with a Russian lawyer, Mr Trump’s son-in-law and eldest son, who had been told in emails that it was part of a Russian government effort to help his father’s campaign.

But Mr Davis later walked back the claims, saying he could not independently confirm that Cohen witnessed Mr Trump’s eldest son, Donald Trump Jr, telling his father about the Trump Tower meeting beforehand.

In the last two weeks, the special counsel has secured the co-operation of Mr Trump’s former campaign chairman, Paul Manafort; signalled that he has obtained all the information he needs from former national security adviser Michael Flynn — who was also a government co-operator; and dispensed with the case of the campaign aide who triggered the Russia probe.

The president has continued a very public battle against the Mueller investigation, repeatedly calling it a politically motivated and “rigged witch hunt”.

He has said he will declassify secret documents in the Russia investigation to show that the investigation was tainted from the start by bias in the Justice Department and FBI.