- Video report by ITV News' Penny Marshall
The FBI is investigating whether Donald Trump's campaign conspired with the Russian government to help win the election.
James Comey, the FBI director, acknowledged - for the first time - the existence of an investigation into Moscow's meddling and crucially whether there was "any coordination between the campaign and Russia's efforts".
Mr Comey's bombshell announcement is hugely significant, confirming an unprecedented scenario where a sitting president's team is under investigation for allegedly colluding with a foreign enemy to propel their candidate to power.
In a further blow to President Trump, Mr Comey also used his appearance before the House Intelligence Committee to publicly refute Mr Trump's claim that former President Barack Obama spied on him.
The president's administration remains defiant, rejecting any suggestion of illicit contact with Russia and standing by the president's baseless accusation that he was placed under surveillance by his predecessor.
- Comey confirms collusion probe
Mr Comey stressed at the start of his testimony that the FBI only commented on active investigations when it was deemed in the public interest to do so.
“This is one of those circumstances,” he said.
He went on to confirm that his agency was "investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts".
Mr Comey also revealed that the investigation into alleged ties between Trump's team and Moscow had been ongoing since July 2016 - five months before Americans went to the polls to elect their new president.
He warned the American people that the "very complex" inquiry would not provide quick answers but promised that his agents would "follow the facts wherever they lead", including examining the possibility that criminal acts had been committed.
He said Russian interference in the election had been "unusually loud" and had been perhaps fuelled by Vladimir Putin's hatred for the Democratic candidate Hillary Clinton.
"They wanted to hurt our democracy, hurt her - help him," he said.
- Wiretap claim rubbished
Mr Comey was asked, once and for all, to dispel President Trump's explosive allegation that Barack Obama placed Trump Tower under surveillance during the election campaign.
He did just that, stating unequivocally that the FBI, National Security Agency and Department of Justice did not have any evidence to support the president's claim, first made in a series of early-morning tweets on March 4.
"With respect to the president's tweets about alleged wiretapping... I have no information that supports those tweets," Mr Comey said.
The FBI director and the National Security Agency director were also pressed on a theory - embraced and elevated by the Trump White House - that Mr Obama asked Britain, America's closest ally, to spy on Mr Trump on his behalf.
The unsubstantiated claim - repeated from the White House podium - infuriated the UK government and led to a rare public denial from British spy agency GCHQ, who branded the suggestion "utterly ridiculous".
NSA Director Admiral Mike Rogers said the claim was completely false and "frustrating to a key ally of ours".
President Trump is now facing increased pressure to retract his accusation and apologise to Mr Obama and Britain.
- White House defiant
Sean Spicer, the White House press secretary, claimed Mr Comey's testimony had changed "nothing", insisting "there's no evidence of a Trump-Russia collusion".
"Investigating it [collusion] and having proof of it are two different things," Mr Spicer told reporters at his daily press briefing.
Asked whether Mr Trump would now back down and apologise for his claimthat Mr Obama had spied on him, Mr Spicer replied: "No...there's moreinformation that needs to be discussed".
President Trump tried to seize the narrative hours before the hearing started, tweeting that the entire "Russian story" was "FAKE NEWS" and urging investigators to switch their focus to the disclosure of classified material to journalists.
During the hearing, Mr Trump used the official Twitter account of the president of the United States to share clips of Mr Comey's testimony,including one which claimed the FBI and NSA had told Congress "that Russia did not influence electoral process".
- Washington reacts
Mr Comey's testimony prompted immediate criticism from Hillary Clinton's closest allies, many of whom still blame him for the Democratic candidate's election defeat last November.
The FBI director famously chose to announce a renewed investigation into Mrs Clinton's emails, just 11 days before the election.
Phillipe Reines, one of her closest confidantes, told ITV News that Mr Comey's decision to disclose that investigation before the election but not the one into possible ties between the Trump campaign and the Kremlin was "maddening".
Mrs Clinton's former press secretary echoed his view.
Senior Democrats called on President Trump to immediately apologise for his baseless wiretap claim against Barack Obama.
Eric Holder, who served as America's top law enforcement officer under President Obama, said Trump had posted "bogus tweets".
Matthew Miller, a former spokesman for the Department of Justice, referenced a clip of the then-candidate Trump inviting Russia to hack his rival's emails back in July 2016 - the same month the FBI started its investigation into Russian interference.