Robert Mueller must resign as special counsel overseeing the Russian investigation, says top Republican congressman

The President has warned the prosecutor would cross a 'red line' if he probes his finances

Robert Mueller addressing leaking in 2013. The former FBI director has been described as 'incorruptible' (Getty)

Trent Franks, of Arizona, has said Mr Mueller, who is currently overseeing a federal probe into possible links between the Trump campaign and Russia’s alleged attempt to interfere in the 2016 election, should resign because of what he claimed was a conflict of interest.

Mr Franks, a member of the House Judiciary Committee, claimed Mr Mueller had a friendship with former FBI Director James Comey, who was fired by Mr Trump in May. Reports suggest Mr Mueller is investigating a possible obstruction of justice by by Mr Trump as a result of the firing.

“Those who worked under them have attested he and Jim Comey possess a close friendship, and they have delivered on-the-record statements effusing praise of one another,” Mr Franks said in a statement.

“No one knows Mr Mueller’s true intentions, but neither can anyone dispute that he now clearly appears to be a partisan arbiter of justice.”

Mr Franks as a reputation as one of the most conservative members on Capitol Hill (AP)

Mr Mueller and Mr Comey worked closely together in the Department of Justice under President George Bush, though it is far from clear they are close friends.

In recent weeks there has been much talk that Donald Trump has been considering firing Mr Mueller, himself a one-time Director of the FBI. He told the New York Times he was angry with his Attorney General, Jeff Sessions, for recusing himself from the investigation, a move he believes then led to the appointment of Mr Mueller.

Mr Trump suggested he was concerned Mr Mueller would not look simply into what happened during the election campaign but also probe his personal finances.

In the interview with the Times, Mr Trump was asked if Mr Mueller’s investigation would cross a “red line” if it expanded to look at his family’s finances.

“I would say yes,” he said. “I think that’s a violation. Look, this is about Russia.”