The most relevant of the 650,000 emails linked to the Clinton campaign could be identified by election day, it was claimed on Monday. However, analysis of their contents and significance will take much longer.

“These kinds of cases by their very nature are painstaking and involve layers of complexity beyond run-of-the-mill investigations,” said Paul Pelletier, a former Justice Department official.

The Clinton campaign on Monday accused Mr Comey of applying a double standard when it comes to the presidential candidates after the FBI director was reportedly opposed to releasing information close to election day about alleged Russian interference in the presidential race.

“A foreign power was trying to undermine the election. He believed it to be true but was against putting it out before the election,” a former official told CNBC.

The CNBC report said that Mr Comey had ensured the FBI's name was not on a document that the US government put out accusing Russia of meddling in the US election.

The FBI declined to comment.

Clinton campaign manager Robby Mook said it was evidence of a "blatant double standard".

Mr Mook called on Mr Comey to "immediately explain this incongruence and apply the same standard to Donald Trump's associates as he has applied to Hillary Clinton's."