Democrats are calling for Donald Trump to be removed from office amid claims that he asked Mr Comey to drop an inquiry into a former national security adviser Michael Flynn.
Mr Trump said: “I hope you can let this go,” according to a memo written by Mr Comey.
The White House has said that the report is "not a truthful or accurate portrayal of the conversation between the president and Mr Comey”.
However if the allegations are true, Democrats have said that Mr Trump could be guilty of obstruction of justice.
Al Green called for “the impeachment of the President of the United States for obstruction of justice” on the House floor yesterday morning.
“It's a position of conscience for me," he said. “This is about what I believe. And this is where I stand. I will not be moved. The president must be impeached.”
Nancy Pelosi, House minority leader, said: “At best, President Trump has committed a grave abuse of executive power. At worst, he has obstructed justice.”
Ted Deutch, Representative of Florida, wrote on Twitter: “Asking FBI to drop an investigation is obstruction of justice. Obstruction of justice is an impeachable offence.”
Donald Trump faces growing calls for his impeachmentq
There are several federal statutes that criminalise obstruction of justice if a person corruptly “obstructs, influences or impedes any official proceeding”.
Under this vague definition, a prosecutor could “probably put together an incredible case” against Mr Trump, said Julie O’Sullivan, a professor at the Georgetown University Law Centre.
“There’s enough to warrant an investigation,” she told PBS, but added: “We can never know what [Trump] was actually thinking.”
Mr Trump’s revelation that he shared intelligence with Russian diplomats has also sparked calls for his removal.
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The White House had denied the accuracy of reports that the President had told the Russians of a terror plot involving laptop computers on trains.
As President I wanted to share with Russia facts pertaining to terrorism and airline flight safety
But Mr Trump confirmed that the exchange had took place when he tweeted: “As President I wanted to share with Russia (at an openly scheduled W.H. meeting) which I have the absolute right to do, facts pertaining to terrorism and airline flight safety.
“Humanitarian reasons, plus I want Russia to greatly step up their fight against ISIS & terrorism.”
Legal blog Lawfare says that as President, Mr Trump is within his rights to disclose classified intelligence.
“Nixon’s infamous comment that ‘when the president does it, that means that it is not illegal’ is actually true about some things. Classified information is one of them,” it said.
Mr Trump admitted to sharing information with Russian diplomats
“The nature of the system is that the President gets to disclose what he wants.”
If any person other than Mr Trump had shared intelligence with Russia, they would “likely be facing a long prison term”, Lawfare adds.
Although Mr Trump did not break any laws, it could be argued that he violated his oath of office if he shared information through carelessness or neglect.
Only two Presidents in history have been impeached – Andrew Johnson and Bill Clinton. Richard Nixon faced impeachment, but resigned before proceedings could get underway.
All three men faced allegations of oath violations. Mr Nixon was also accused of abusing his power as President.