Donald Trump reportedly asked James Comey to drop the FBI inquiry into Michael Flynn's ties with Russia.
US media is reporting this allegation based on a memo which was supposedly written by the former FBI director himself.
The memo is believed to have been written immediately after a meeting in which the US President urged Comey to drop the inquiry into the former national security advisor's links to Russia.
The meeting supposedly took place one day after Flynn had resigned from his position, after it emerged he had misled Vice President Mike Pence about his conversations with Russia during the election campaign.
The White House has issued a statement to flatly deny this allegation, saying Trump had never asked Comey or other individuals to "end any investigation."
Meanwhile, a Republican congressman has reportedly called for the FBI to hand over any important documents within the week.