Former Trump campaign chairman Paul Manafort has sued special counsel Robert Mueller and the Justice Department over the probe into Russian interference in the 2016 presidential election.

The lawsuit by Mr Manafort, filed in federal court in Washington on Wednesday, claims prosecutors overstepped their bounds by charging him for conduct that he says is unrelated to the Russian investigation.

It is the most direct challenge to date to Mr Mueller’s legal authority and the scope of his mandate as special counsel.

It comes amid Republican allegations of partisan bias among members of Mr Mueller’s team, which for months has been investigating whether the Trump campaign coordinated with Russia to influence the outcome of the US election.

Mr Manafort was indicted in October on charges related to his lobbying work on behalf of a Russia-friendly Ukrainian political party. He has pleaded not guilty.

He alleges in his lawsuit that the investigation into “decade-old business dealings” is “completely unmoored” from the mandate Mr Mueller was given when he was appointed in May by Deputy Attorney General Rod Rosenstein.

“Those alleged dealings had no connection whatsoever to the 2016 presidential election or even to Donald Trump. Nor were they uncovered in the course of the Special Counsel’s probe into President Trump’s campaign,” the suit alleges.

The lawsuit also argues that Mr Rosenstein’s order appointing Mr Mueller was overly broad and arbitrary.

Mr Manafort’s lawyer, Kevin Downing, did not immediately return a call seeking comment.

A spokesman for Mr Mueller’s office, Peter Carr, declined to comment.

– AP