In a step that confirmed a sitting president and his associates were part of an espionage investigation, FBI director James Comey said it was rare for the bureau to comment on the existence of an ongoing probe.

But among a backdrop of leaks, innuendo and allegations of “fake news”, Mr Comey said he had been authorised to confirm the outline of the investigation, even if he was not permitted to provide many other details.

“The FBI, as part of our counterintelligence effort, is investigating the Russian government’s efforts to interfere in the 2016 president election,” he said.

“That includes, investigating the nature of any links between associates of the Trump campaign and the Russian government and whether there was any coordination between the campaigning the Russian effort.”

He added: “I cannot say more about what we are doing and whose conduct we are examining.”

Mr Comey also said he had not been shown evidence that supported Mr Trump’s claim to have been wiretapped by his predecessor in the White House, Barack Obama.

Mr Trump has repeatedly denied any links between his campaign and the Russian government. The head of the congressional committee, before which Mr Comey testified, said it had so far found no evidence of collusion.

But the mere public confirmation that Mr Trump and his officials are part of an ongoing counterintelligence probe is likely to become a millstone around the neck of his administration, already struggling for traction in its first 100 days.

Mr Trump’s supporters will likely brush off the probe as nothing more than a politically motivated attack. Ahead of Monday’s hearing by the House Intelligence Committee, Mr Trump said on Twitter: “[Former Director of National Intelligence] James Clapper and others stated that there is no evidence Potus colluded with Russia. This story is FAKE NEWS and everyone knows it!”

Yet among opponents of Mr Trump, knowledge that a probe has been ongoing since last July will add to the anger of those who think Mr Comey acted unfairly in regard to Hillary Clinton.

Why, many people asked, was the FBI director willing to make an 11th-hour intervention during the election campaign by revealing the probe into Ms Clinton’s use of a private email server had been reopened, but not that her opponent was also being investigated?

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Senator Chuck Schumer of New York, the Democratic leader in the Senate, said of the probe: “The possibility of coordination between the Trump campaign and Russian officials is a serious, serious matter. The investigation must be fair, independent, and impartial in every way, and the FBI must be allowed to follow the facts wherever they may lead.”

Mr Comey appeared to try and see off any suggestions that he had not acted fairly. He said members of congress should not make a comparison between his lengthy public comments on the Clinton inquiry, arguing that they concerned “details of a completed investigation”.

Mr Comey, who testified alongside Admiral Michael Rogers, director of the National Security Agency, said it remained the view of the intelligence community that Russia had interfered as part of an effort to try and tilt the election in favour of Mr Trump.

“I think that was a fairly easy judgement for the [intelligence] community,” he said. “Putin hated Secretary Clinton so much, that the flip side of that coin was he had a clear preference for the person running against the person he hated so much.”

Asked about Mr Trump’s allegation that Mr Obama had wiretapped Trump Tower during the election, Mr Comey and Mr Rogers said they had seen no information to support such a claim. The White House last week said Mr Trump stuck by his allegation.

Mr Rogers also said there was no evidence to support another claim that was made by Mr Trump’s spokesman, Sean Spicer – namely that British intelligence had carried out the electronic surveillance at the behest of Mr Obama.

“I’ve seen nothing on the NSA side that we engaged in such activity, nor that anyone engaged in such activity,” he said.

The US government alleged last summer that Russian intelligence had tried to interfere with the presidential election, in an apparent attempt to help Mr Trump. This included hacking into the emails of senior members of Ms Clinton’s team and the Democratic National Committee.

Mr Trump long rejected the claim that Russia had came to his assistance. Moscow also denied the allegations. In late December, Mr Obama expelled 35 Russian diplomats in punishment for the alleged interference. Vladimir Putin surprised many by deciding not to reciprocate.

Mr Trump has continued to insist there was collusion between his campaign and Russia, and US intelligence has yet to provide any evidence to support such an allegation.

Mr Comey was asked whether he had yet uncovered anything to support such a conclusion. He responded by saying that he would not be able to comment until the probe was concluded – something he said would not happen quickly.