The British connection in the investigations into Donald Trump and the Kremlin will be in focus at a grand jury session this week with the appearance of a witness with ties to Nigel Farage and others in the Brexit campaign.

Ted Malloch, an American academic living in the UK, has been detained by the FBI and questioned about his involvement in the Trump presidential campaign. He was also asked about his alleged links to Julian Assange and whether he had met the Wikileaks founder at his refuge in the Ecuadorian embassy in London. Mr Assange has been accused of playing a part in disseminating Democratic Party emails, allegedly hacked by the Russians, which damaged Hillary Clinton’s presidential campaign and helped that of Mr Trump.

The FBI agents, according to Mr Malloch, had also been keen to find out about his association with Roger Stone, a long-time supporter and confidant of Mr Trump, who had claimed in the past that he had met and had dinner with Mr Assange in London and that the Kremlin had been responsible for the Democrat hacking. Mr Stone is also reported to have known that Wikileaks were preparing to release the emails before they became public.

A subpoena was served on Mr Malloch and his mobile telephone seized by the FBI when he flew to Boston from London at the end of last month. At the same time, he says, he was warned that it was a felony offence carrying a term of imprisonment to lie to the FBI.

Mr Malloch is due to testify on Friday 13 April to a grand jury as part of the investigation by Robert Mueller, the special counsel investigating Russian interference in the US presidential election which allegedly helped Mr Trump to the White House.

The UK has been increasingly featuring in the Mueller investigation. Alex Van der Zwan, living in London, became the first person to be jailed due to the inquiry. The 33-year-old lawyer, who had previously worked with Mr Trump’s former campaign manager, Paul Manafort, was sentenced to 30 days in prison and fined $20,000 (£14,000) after pleading guilty to lying to the FBI.

The start of the FBI investigation in July 2016 into the Russia allegations can now be traced back to information which came from London even before the former MI6 officer, Christopher Steele, produced his explosive Trump dossier.

In the summer of 2016 George Papadopoulous, a young Trump foreign policy advisor, confided to Alexander Downer, the Australian high commissioner to the UK, at west London bar the Kensington Wine Rooms that the Russians held thousands of emails which would embarrass Hillary Clinton. Australian intelligence officials passed on details of the conversation to their American counterparts after the emails began to be published by Wikileaks. Mr Papadopoulous, it emerged, has made contact with Moscow through Roger Mifsud, a Maltese academic, then also based in London.

Mr Papadopoulous has pleaded guilty to lying to the FBI and is now cooperating with Mr Mueller’s investigators.

The revelation by Mr Papadopoulous was followed by another key strand in the inquiry, the dossier on Mr Trump and Russia put together by Mr Steele, which was passed on to the FBI director James Comey by senator John McCain.

Mr Malloch, it was claimed, was going to be appointed as ambassador to the European Union if Mr Trump became president. His outspoken anti-Brussels views alarmed some senior European officials, including Manfred Weber and Guy Verhofstadt, who publicly raised doubts about his suitability for the post.

The stance of the EU officials drew the ire of Arron Banks, the former Ukip funder and co-organiser of the Leave.EU campaign. His Westmonster website described the objections to Mr Malloch’s supposed appointment, as “EU nationalists attempting to block Trump’s EU Ambassador”.

Nigel Farage, who interviewed Mr Malloch on his LBC show, described him as “the man tipped to be Trump’s EU ambassador” and wrote an afterword for a book by Mr Malloch called Hired: An Insider’s Look at the Trump Victory. Mr Farage tweeted after a television appearance by the academic: “Ted Malloch is the boy!” Mr Farage also met Mr Malloch in Brussels, with The Spectator magazine reporting it had been passed “a snap of Farage meeting this morning with Trump’s soon-to-be man in Brussels, professor Ted Malloch”.

But serious doubts were raised over Mr Malloch’s credibility after a Financial Times investigation discovered that some of the claims about himself were untrue. He had not, as he had said in his autobiography, been “knighted” into the Sovereign Order of St John by the Queen, nor has he been called a “genius” by Margaret Thatcher.

Mr Malloch was not a fellow of an Oxford College, as he had also claimed and had not been an “ambassadorial level” official at the UN. He had also, it was discovered, declared himself bankrupt in 2013. The Trump administration then told The Wall Street Journal that Mr Malloch was not and never had been, considered for the post of European Union ambassador.

Mr Malloch, however, has told associates that he was able to organise a visit to Trump Tower in New York by Mr Farage, Mr Banks and a another Brexit campaigner, Raheem Kassam. He has known Mr Trump since 1981, he says, and claims that he was involved in arranging Mr Trump’s visit to Brussels and Italy.

Mr Malloch, who has a book coming out next month, referred questions to his publishers. They produced a long statement by him in which he described his detention and questioning by the FBI at Boston airport on 27 March.

“After exiting the plane, I was escorted to a special line for passport control. There, I was formally detained and asked to wait, along with my wife who was travelling with me. I was then escorted to another building and into a secure conference room where two young FBI agents introduced themselves to me. They said I was being detained to answer questions regarding the Department of Justice special counsel probe and showed me their identification,” he said.

“They seemed to know everything about me and had my colour photograph and personal details and said in intimidating ways, that it was a felony to lie to the FBI. I stated that I realised that and I would readily, in fact gladly, cooperate with them … The questions got more detailed about my involvement in the Trump campaign (which was informal and unpaid); whom I communicated with; whom I knew and how well – they had a long list of names…”

Mr Malloch wanted to stress that he had not met Mr Assange and had only met Roger Stone in the company of other people.

It may well be that Mr Malloch’s exaggerations about his importance had drawn him to the attention of the FBI and Robert Mueller. But it is now clear from the scope and scale of the investigation that it is continuing to spread with interrogations and detentions following from witness interviews.

Mr Stone had, in the past, claimed that as well as meeting Mr Assange, he had a source in London who was a “mutual friend” of him and Mr Assange and this source had told him about the impending Wikileaks release of the Democratic Party emails. Mr Assange announced in June 2016 that Wikileaks had the Clinton emails and that they would be published soon.

A month later Mr Stone said he believed the Russians were probably behind the hacking. He said at the time “the reason that the Russians are probably leaking this information is because they don’t want a nuclear war. [Hillary Clinton] is bent on a war that benefits her donors and the multinational corporations and the defence contractors.”

Mr Stone now denies meeting Mr Assange and also denies that the Russians were behind the email hacking.

Glenn Simpson, whose investigations firm hired Mr Steele to produce the Trump report, told a US Congressional inquiry in January that Mr Farage was a more frequent visitor to Mr Assange at the Ecuadorian embassy and had passed data on to Mr Assange “in a thumb drive”.

Mr Farage has stated that he only met Mr Assange once, in March 2017, for journalistic purposes and on no other occasion. The Twitter account of Wikileaks said “the question was about what kind of data. Game of Thrones or emails? 2016 or 2017? Simpson answers with a diversion.”

Mr Farage told The Independent: “Yes, I met Ted Malloch a few times. I read his book and I thought what he had to say about the death of Davos Man was fascinating. He is an East Coast Conservative with interesting views.

“Other than that I do not know about him and the Trump campaign and why he has been questioned and is before a grand jury or why there should be questions about me.

“I met Julian Assange just once. I went there in a journalistic capacity because like you I wanted to find out about the emails, no real answer was forthcoming. It is nonsense to say that I had met him secretly. Do you think one of the best known faces in the country can go into the embassy without people noticing?”