By Luke Harding, Stephanie Kirchgaessner & Nick Hopkins @ TheGuardian.com, April 13

Britain’s spy agencies played a crucial role in alerting their counterparts in Washington to contacts between members of Donald Trump’s campaign team and Russian intelligence operatives, the Guardian has been told.

GCHQ first became aware in late 2015 of suspicious “interactions” between figures connected to Trump and known or suspected Russian agents, a source close to UK intelligence said. This intelligence was passed to the US as part of a routine exchange of information, they added.

Over the next six months, until summer 2016, a number of western agencies shared further information on contacts between Trump’s inner circle and Russians, sources said.

The European countries that passed on electronic intelligence – known as sigint – included Germany, Estonia and Poland. Australia, a member of the “Five Eyes” spying alliance that also includes the US, UK, Canada and New Zealand, also relayed material, one source said.

Another source suggested the Dutch and the French spy agency, the General Directorate for External Security or DGSE, were contributors.

It is understood that GCHQ was at no point carrying out a targeted operation against Trump or his team or proactively seeking information. The alleged conversations were picked up by chance [....]

One source called the British eavesdropping agency the “principal whistleblower”.

The Guardian has been told the FBI and the CIA were slow to appreciate the extensive nature of contacts between Trump’s team and Moscow ahead of the US election. This was in part due to US law that prohibits US agencies from examining the private communications of American citizens without warrants. “They are trained not to do this,” the source stressed [.....]

According to one account, GCHQ’s then head, Robert Hannigan, passed material in summer 2016 to the CIA chief, John Brennan. The matter was deemed so sensitive it was handled at “director level”. After an initially slow start, Brennan used GCHQ information and intelligence from other partners to launch a major inter-agency investigation.

In late August and September Brennan gave a series of classified briefings to the Gang of Eight [.....]