A hand operates above a laptop's keyboard in London | Andy Rain/EPA British spies discovered Trump team’s alleged links to Russia: report UK intelligence began sending information about Trump campaign officials’ contacts with Russia from 2015.

Britain's spy agency GCHQ was the "principal whistleblower" in the investigation into alleged collusion between U.S. President Donald Trump's campaign team and Russian intelligence operatives, the Guardian reported Thursday.

According to the report, GCHQ began passing information about contacts between Trump campaign officials and Russian operatives in late 2015. Agencies from Australia, Germany, Estonia, France, the Netherlands and Poland also passed on information until summer 2016.

Sources said the U.S. was slow to respond to the intelligence it was receiving regarding possible Russian meddling in the U.S. presidential race.

“It looks like the [U.S.] agencies were asleep,” a source told the Guardian. “They [the European agencies] were saying ‘There are contacts going on between people close to Trump and people we believe are Russian intelligence agents. You should be wary of this.’"

GCHQ refused to comment on its role in the ongoing FBI-led investigation.

The report said the information was gathered from routine surveillance of Russian operatives, not the Trump campaign team. The Trump administration had earlier accused the British spy agency of "wiretapping" Trump Tower, in collusion with former president Barack Obama.