James Comey, the ousted FBI chief, accused the White House of lying about him and the agency in gripping testimony to Congress that is likely to undermine President Donald Trump and fuel the probe into alleged collusion between his team and Russia.

The allegations came at a high-stakes Senate hearing where Mr Comey described Mr Trump as untrustworthy, at one point asserting "Lordy, I hope there are tapes” of his private conversations with the president, to demonstrate the veracity of his testimony.

His testimony could prove pivotal to the multiple probes into whether members of Mr Trump’s inner circle had improper contacts with Russia during the presidential race.

Mr Comey revealed that Michael Flynn, a former campaign aide who became Mr Trump’s first national security adviser, was under criminal investigation when the president privately urged him to drop any FBI probe into the retired general.

Mr Comey said he was "stunned” when Mr Trump made the request in a private Oval Office meeting, saying: "I took it as a direction that this is what he wants me to do.”

Mr Comey’s understanding of the conversation could prove critical to the justice department’s Russia investigation, which is now being headed by special prosecutor Robert Mueller.

Mr Comey said that at the time of the meeting, Mr Flynn was "in legal jeopardy”. He also suggested that Mr Mueller would examine whether Mr Trump’s remarks were an effort to obstruct justice.

"I don’t think it’s for me to say whether the conversation I had with the president was an effort to obstruct,” Mr Comey said. "I took it as a very disturbing thing . . . but that’s a conclusion I’m sure the special counsel will work towards, to try and understand what the intention was . . . and whether that’s an offence.”

Although Mr Comey had detailed the contents of his meetings and phone calls with Mr Trump in testimony published on Wednesday, he used his opening statement to vent anger at the White House, saying he had become "increasingly concerned” with the changing explanations the Trump administration gave to justify his firing last month.

While his account of the private meetings was detailed and dispassionate, he showed uncharacteristic emotion when he addressed Mr Trump’s claim in the wake of his sacking that he had mismanaged the FBI and was unpopular within its ranks.

"The administration then chose to defame me and, more importantly, the FBI by saying that the organisation was in disarray . . . that the workforce had lost confidence in its leader,” Mr Comey said. "Those were lies, plain and simple.”

Mr Comey confirmed he had told Mr Trump on several occasions that the president was not a target of the FBI investigation while he was in office.

The president’s lawyer, Marc Kasowitz, said Mr Comey had confirmed that Mr Trump "was not under investigation” as part of any Russia investigation. During his appearance on Capitol Hill, however, Mr Comey made clear that his assurances to Mr Trump about the president not being under investigation were based on the state of events at that time.

Mr Comey also told Congress that he had intentionally leaked his post-meeting notes of his conversations with Mr Trump to a friend with instructions to share them with the media, saying he took the decision after Mr Trump had tweeted about the possibility that he had secretly taped their meetings in the White House.

"I thought it might prompt the appointment of a special counsel,” Mr Comey testified.

The Trump camp seized on the admission. The president’s son, Donald Trump Jr, said it was clear evidence his father was right about leaking aimed at undermining his administration.

"Washington DC at its finest,” his son tweeted. "This is what America rallied against, but the DC elite don’t get it or know any other way.”

Mr Trump did not comment, or tweet, about the testimony. But in a speech to a religious forum, he cited the Old Testament, before saying: "The entrenched interests . . . in Washington will do everything in their power to try and stop us from this righteous cause . . . They will lie, they will obstruct, they will spread their hatred and their prejudice, but we will not back down from doing what is right.”

Mr Comey said he documented his private conversations with Mr Trump — which he had not done with predecessors Barack Obama or George W Bush — because he was concerned the president might mischaracterise their content. "I was honestly concerned that he might lie about the nature of our meeting,” he said.

In prepared remarks released the day before his hearing, which Mr Comey did not repeat in front of the committee, he detailed nine private conversations — three in person and six over the telephone — he held with Mr Trump over four months, compared to only two with Mr Obama over the course of the previous three years.

Mr Comey described a private dinner in the White House on January 27 where the president told him he wanted "loyalty” from his FBI chief. "Common sense told me he is looking to get something for granting my request to stay in the job,” he said.

Mr Kasowitz disputed that account, saying Mr Trump had never told Mr Comey that he wanted loyalty.

Mark Warner, the top Democrat on the committee, said Mr Trump’s phone calls, in which the president asked Mr Comey to "lift the cloud” of the Russia investigation, were in "violation of clear guidelines put in place after Watergate to prevent any whiff of political interference by the White House into FBI investigations”.

Agencies