LONDON—The extent of Russia’s interference in the 2016 votes for Trump and Brexit has been investigated by intelligence agencies, congressional and parliamentary inquiries, the FBI and special counsel Robert Mueller’s office for more than a year.

For much of that time, a reporter in England has been in possession of extraordinary details about Russia’s cultivation and handling of Brexit’s biggest bankroller. Arron Banks was secretly in regular contact with Russian officials from 2015 to 2017, according to a cache of emails apparently not seen in those Transatlantic investigations until they were published in Britain on Sunday.

Banks, who ran the Leave.EU campaign group, was one of the first foreign political figures to visit Donald Trump—accompanying Nigel Farage to Trump Tower—soon after the shock presidential election of 2016. Farage is reportedly a “person of interest” in the FBI’s Trump-Russia investigation.

The Sunday Times reported that Banks admitted passing over contact details for members of the Trump transition team to Russian officials and meeting with the Russian ambassador in London just three days after their Trump Tower summit.

Their package came complete with a commentary from Oakeshott herself, in which she expressed her shock at the revelations. “I was very surprised by what I found, which conflicted with the public accounts of the relationship with the ­Russian embassy,” she wrote. “Suddenly the Russian embassy in ­London had a potential back channel to the White House.”