However analysts suggested that if Mrs Clinton sacks Mr Comey she will bring the focus back to the email scandal.

Senior Democrats and Republicans yesterday refused to express confidence in Mr Comey.

Dianne Feinstein, the top Democrat on the Senate Select Committee on Intelligence, said Comey "unfairly hurt the campaign of one candidate and changed the tenor of this election."

Jason Miller, Mr Trump’s senior communications adviser, said: "It is clear that the system is rigged. It is clear that this investigation has not been handled very well from the beginning."

What now for James Comey?

Can the head of the FBI be fired?

Yes. Under US legislation that was enacted in 1968, the president nominates the director of the FBI and has the authority to remove him.

Has it happened before?

Only one FBI director has been fired in history. William S Sessions, who was appointed by President Ronald Reagan in 1987, was sacked by Bill Clinton over a series of ethics violations.

They included disguising private holidays as law-enforcement business and using a chauffeured government limousine for personal transport. Mr Clinton repeatedly asked him to resign but he refused and was sacked in 1993.

Could Mr Comey be sacked then?

Mr Comey's intervention, on the eve of the election, has provoked fury among both Democrats and Republicans. Mrs Clinton directly criticised the decision to re-open the email investigation, while senior Democrats have accused Mr Comey of "partisan actions".