Michael Flynn, US President Donald Trump's short-lived national security adviser, has stopped sharing information with the president's legal team, it has been reported – giving rise to speculation that Flynn could be preparing a plea deal with Robert Mueller.

Mueller is investigation Russian interference in the US election, and whether there was any collusion between Trump's campaign and operatives from Moscow.

Robert Mueller is investigating Russian interference in the US election, and Flynn could be about to cooperate. Photo / Getty
Robert Mueller is investigating Russian interference in the US election, and Flynn could be about to cooperate. Photo / Getty

Two people – Paul Manafort, the former campaign chairman, and Rick Gates, Manafort's right-hand man and a senior aide to the Trump campaign – have already been charged in connection with Mueller's investigation.

The pair were indicted at the end of last month on charges including money laundering and conspiracy against the United States, and both pleaded not guilty.

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Flynn, who was fired by Barack Obama and who Obama subsequently warned Trump against hiring, was long speculated to be in Mueller's line of sight.

The New York Times said that four separate sources told them Flynn's lawyers have informed Trump's team that they can no longer share information.

Defence lawyers frequently share information during investigations, but they must stop when doing so would pose a conflict of interest.

Former National Security Adviser Mike Flynn listening to US President Trump. Photo / Getty
Former National Security Adviser Mike Flynn listening to US President Trump. Photo / Getty

It is unethical for lawyers to work together when one client is cooperating with prosecutors and another is still under investigation.

The move by Flynn's lawyers is not seen as proof that Flynn is cooperating with Mueller – but Trump's lawyers are reported to believe that Flynn has, at least, begun discussions with Mueller about cooperating.

Flynn served as national security adviser from Trump's January 20 inauguration until February 13, when he was forced to resign and admit lying to Mike Pence, the vice president, over his meetings with the then-Russian ambassador Sergey Kislyak.

Flynn has a long history of dealings with Moscow, and was famously seated at Vladimir Putin's table during a December 2015 dinner.