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.

Isabel Oakeshott, a former Sunday Times journalist who ghost-wrote Banks’ book, The Bad Boys of Brexit, was granted access to his emails in the summer of 2016 in order to help draft the diaries. The book mentions one meeting at the Russian embassy which has been the focus of great interest ever since, especially amid questions about where Banks’ sourced the multi-million pound funding of Brexit. He has denied the money came from Russia.

Oakeshott says she did not discover the stunning extent of Banks’ true dealings with Russia until last year. Even then, she decided not to publish saying she wanted to wait until the publication of her next book White Flag? in August. It is unclear whether the Electoral Commission’s investigations into Banks’ financing of the Brexit campaign would have been completed by August.

Oakeshott was keen to keep her treasure trove of Brexit/Russia revelations for her book launch, but she has not merely kept out of the debate about the legitimacy of the Brexit campaign. Describing herself as “a long-standing Brexit supporter,” who is close to Farage and Banks, Oakeshott has become a regular TV pundit shooting down “conspiracy theories” about the validity of the Brexit vote amid claims of Russian influence or reports about Cambridge Analytica’s disputed involvement.

Three months ago she confronted The Observer’s Carole Cadwalladr live on the BBC after Cadwalladr’s stories uncovering the misuse of tens of millions of Facebook profiles by Cambridge Analytica, which was linked to the Trump campaign and Leave.EU.

Cadwalladr, who has spent the last two years investigating the nexus of Farage, Banks, Trump, Cambridge Analytica and Russia, raised concerns about the validity of the Brexit vote. When the presenter asked Oakeshott about her relationship with Banks, she said: “There just isn’t a conspiracy here, Carole, I just feel like you’re chasing unicorns.”

Oakeshott’s attitude apparently changed on Friday when she learned that Cadwalladr—along with Peter Jukes—was preparing another story for Sunday.

An email, seen by The Daily Beast, was sent to Banks at 11.57am on Friday by Cadwalladr advising him that The Observer had obtained copies of his emails which laid bare the scale of his interactions with Russia. They appeared to show that he and Leave.EU colleague Andy Wigmore had multiple meetings with high-ranking Russian officials, that Banks visited Moscow in February 2016, and that he had been introduced to a Russian businessman by the Russian ambassador who allegedly offered him a multibillion dollar investment opportunity in Russian goldmines.

Banks did not respond to the email until 10.30pm that night, saying he was out of the office and could not respond until Monday.

Within hours, Oakeshott was in touch with Cadwalladr, however. At first she accused The Observer of hacking her archive and stealing the emails—an allegation the reporters deny—but by late afternoon on Saturday she had entered into a discussion about cooperating with The Guardian/The Observer if they agreed to hold the story until Monday.

By then, a team at The Sunday Times, where Oakeshott used to work, was in full swing producing their own version of the stunning story which they managed to break before The Observer late on Saturday.

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.”

Oakeshott has not responded to questions from The Daily Beast, including whether she has passed the emails to the FBI, the Mueller probe or Britain’s Electoral Commission.

Jukes said he was concerned that the information may not have reached the ongoing inquiries in time. “There’s every indication that Isabel Oakeshott was planning to hold back revealing this explosive material until her book was published in August,” he said. “With an Electoral Commission investigation into Banks’ financing of Brexit underway since November, you would have thought that the public interest of this story was more important than keeping the scoop for a book.”