A mysterious academic who allegedly acted as a link between Donald Trump's presidential campaign and the Kremlin is "missing and may be dead", according to court papers.

Joseph Mifsud, 57, became embroiled in scandal after he was said to have offered "dirt" about the Democratic candidate Hillary Clinton to a Republican aide at a London hotel.

The Maltese professor, based at the University of Stirling in Scotland until last year, has previously denied suggestions that he was a Russian agent saying: "Secret agent! I never got a penny from the Russians. My conscience is clean."

But last week he was named in papers lodged with a New York court by the Democratic National Committee – the Democratic Party's governing body – which is suing Russia, the Trump campaign and WikiLeaks for allegedly interfering in the 2016 election, according to the Daily Mail.

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The documents said all the defendants in the case have been served with the complaint, "with the exception of Mifsud (who is missing and may be deceased)".

Lawyers did not elaborate.

Mifsud's fiancée, who gave birth to his daughter this year, is based in Ukraine. She reportedly said she hasn't seen or heard from him for months.

Last week, George Papadopoulos, the former Trump adviser to whom Mifsud is said to have made his offer, was sentenced to 14 days in prison for lying to the FBI during its investigation into election meddling.

He is the first former campaign aide to be sentenced in special counsel Robert Mueller's Russia probe, which is also investigating any possible collusion between the Trump campaign and Moscow.

It was claimed Mifsud, a former honorary director of the London Academy of Diplomacy, also told Papadopoulos he could set up a meeting with Vladimir Putin ahead of the 2016 election.

Authorities were alerted after Papadopoulos told an Australian diplomat during a drinking session in a London pub about his meetings with Mifsud.

Mifsud was lauded by the University of Stirling as part of its "diplomacy A-team", a globetrotting ambassador "flying the flag" for the Scottish institution. Allegations contained in federal indictments against Papadopoulos say that while Mifsud was working with Stirling, he was offering to cultivate his "substantial connections with Russian government officials" to deliver "thousands of emails" that would damage Mrs Clinton.

Mifsud has a colourful career history. He is said to have left a job at the University of Malta under a cloud in 2007, before becoming president of a university in Slovenia.

He quit that job, disputing claims he fiddled expenses worth £34,320 (NZ$67,839). He was once described as 'Ambassador Mifsud' but although he worked for six months in the private office of the Maltese foreign minister, he was never a diplomat.

He has been, however, an adviser to Malta's government on its entry to the EU, a guest lecturer at seminars around the world, a speaker on Capitol Hill, and an expert on Brexit for Russian MPs.