The investigation into the leak of the confidential dispatches of Sir Kim Darroch, the British ambassador to Washington, is progressing faster than expected with extensive evidence being collected about a number of suspects, according to security sources.

Scotland Yard’s Anti-Terrorist Branch, GCHQ and another government security agency have been involved in the investigation which is believed to have rapidly narrowed down the identity of suspects, despite Sir Kim’s emails having been made potentially available to more than a hundred people.

Officials close to the inquiry believe “there was a degree of orchestration” behind the theft of emails and their subsequent publication in a newspaper, and are considering “all motives, including political ones”, they say.

A 19-year-old freelance journalist has claimed in the Mail on Sunday, the newspaper which published Sir Kim’s emails, that he was a conduit for the leak. He had obtained the material, he said, while talking to civil servants for a research project.

Steven Edginton works as a “digital strategist” in Nigel Farage’s Brexit Party, and is also “chief digital strategist” at the Leave Means Leave group.

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He has contributed to Brexit Central’s website and worked for pro-leave website Westmonster, and the right-wing TaxPayers’ Alliance pressure group.

Sir Kim’s dispatches were highly critical of the chaotic and dysfunctional nature of Donald Trump’s administration, and the US president’s relationship with truth.

It led to a furious reaction from Mr Trump, who kept up a barrage of insults towards the ambassador for a number of days along with demands for him to be recalled from Washington.

Sir Kim resigned after Boris Johnson, in a debate with Jeremy Hunt in the contest for the Conservative party leadership, repeatedly failed to offer an assurance that he would keep the ambassador in place if he became prime minister.

Boris Johnson opposes prosecution of press over Kim Darroch leak

Sir Kim had been scathingly criticised by leading Brexiters, including Mr Farage, who demanded he be replaced by someone is pro-Brexit and also sympathetic to Mr Trump.

Asked about Mr Edginton and the leak, Mr Farage told The Independent: “I know him, he is a young man, a freelance and he has done what every other journalist would have done given such stuff.

“I did not know his part in the leak until I read about it. He works for us, I knew he had also some other freelance activities. I can’t understand all this fuss, after all what was passed to him wasn’t a national secret, it wasn’t the nuclear codes, so I don’t see what all this is about.”

Asked whether whoever stole the ambassador’s emails and passed them on should be prosecuted, Mr Farage responded: “I don’t know what the terms of his contract were, whether it was covered by the Official Secrets Act. But I think it does show how fed up many people are about the way the civil service has been politicised over Brexit, there is real anger about this. There have been plenty of leaks, I think the establishment is totally overreacting to this particular one.”

Mr Johnson, after facing severe criticism, including from a huge number of Tory MPs, for failing to stand by the ambassador, declared his wish that whoever was guilty of the leak “should be run down, caught and eviscerated”.

Trump wishes Sir Kim Darroch well after resignation

Mr Trump meanwhile, in a volte-face of the type for which he has become known, asserted later that the ambassador had said “very good things” about him and was “sort of referring to other people” when criticising the White House.

Mr Edginton tweeted in April this year “after the establishment have betrayed Brexit, we are currently working on the fight back. All efforts are being made”. Another tweet said: “Ministers are simply fed documents by Remainer civil servants and without question follow their advice and order.”

He insisted in his newspaper article, however, that there was no political motivation to him passing on the documents. It was, he wanted to stress, “simply an honest journalistic endeavour … As a 19-year-old freelance journalist with a passion for politics, I was looking for a big project through which to develop my career”, leading him to speak to “current and retired civil servants” and ultimately gain access to the emails.

A senior security source refused to comment on Mr Edginton’s claims, saying it “did not materially change” the course of the investigation.

One person, according to officials, was primarily responsible for stealing the emails and although this may have been an “opportunistic” theft, the inquiry is looking into the alleged plan involving a number of people in the way it was then disseminated.

John Bercow describes Sir Kim Darroch as an 'outstanding public servant'

In his article, Mr Edginton said of Sir Kim’s emails: “I was shocked by the brutal language from a supposedly impartial diplomat ... Sir Kim’s comments about Trump were jaw-dropping and suggested a lack of impartiality.”

But he went on to observe, rather confusingly: “Sir Kim was simply articulating what many in Washington and Whitehall have said about the president and his advisers since he took office.”

Mr Edginton said he did not regret “my role in the story”, although he said the furore it has generated has caused him to lose weight and struggle to get to sleep. He was now “suspicious of everything”, recounting how “last week I was eating my lunch near the Houses of Parliament when I spotted a middle-aged man dressed as a tourist taking pictures of me. He then furtively ducked behind a tree before, I think, getting into a white van. Was it the security services? Am I being followed? I will probably never know...”

Theresa May on Kim Darroch's resignation: 'It is a matter of great regret'

According to pressure group Hope not Hate, Mr Edginton has been associated with right-wing group Turning Point UK (TPUK), a pro-Trump organisation in America which has been endorsed by a number of Brexiteers, including Priti Patel and Jacob Rees-Mogg.

However, another major pro-Brexit figure, Aaron Banks, described a leading member of TPUK, John Mappin of jewellery family Mappin & Webb, as “a total fruit loop”.

Mr Banks, who, it was recently alleged, was providing Mr Farage with a furnished Chelsea home, a car and driver, and money to promote him in America, claimed in his book Bad Boys of Brexit Mr Mappin had told him “he’s trying to launch a super-powered brain-control system that requires delivery facilities in 50 languages in every major city”.