They say it was "incidental collection", much like the kind that was captured by USA agencies on Trump associates.
"They [the European agencies] were saying: 'There are contacts going on between people close to Mr Trump and people we believe are Russian intelligence agents".
The Guardian said it understood that GCHQ at no point conducted any targeted operation against Mr Trump or members of his team and that the agency had not been proactively seeking information.
Despite the tip from the British agency, shared during routine briefings, the Federal Bureau of Investigation and CIA were slow to delve into the information, since American agencies are "trained not to" look through citizens' private communications if there is no warrant, said a source who called GCHQ the "principal whistleblower".
"I have been authorized by the Department of Justice to confirm that the Federal Bureau of Investigation, as part of our counterintelligence mission, is investigating the Russian government's efforts to interfere in the 2016 presidential election", Comey told members of Congress on March 20, according to NPR.More news: Project Scorpio Dev Kit Images Revealed
One of the lawmakers, Senate Minority Leader Harry Reid, wrote two letters to FBI Director James Comey between August and October urging him to go public with the information.
The sources reportedly added that in the first half of 2016, a number of western countries, including Germany, Estonia, Poland, Australia, the Netherlands and France, also passed on information about alleged contacts between Trump's team and Moscow to the United States. Trump was not named in the warrant, but three of his associates were the subject of the inquiry.
The Trump administration had earlier accused the British spy agency of "wiretapping" Trump Tower, in collusion with former president Barack Obama.
"Fox News can not confirm Judge Napolitano's commentary".
The comment prompted a near-immediate reaction from GCHQ, which released a rare public statement categorically denying any involvement and calling the accusation "utterly ridiculous".