The international confidence trickster who gave £2.4m of fraudulently obtained money to the Liberal Democrats is believed to be hiding in the Caribbean with access to a multimillion pound stolen fortune, inquiries by the Guardian have found.

Michael Brown, a 44-year-old Glasgow-born fugitive and the party's most generous donor, has been living under a false British identity and moved between tropical islands as recently as last year, according to private investigators.

He disappeared in June 2008 while awaiting trial for false accounting and theft and was sentenced in his absence to seven years in jail.

Police believe he has escaped with up to £20m of stolen money after analysing bank accounts attached to his fraudulent company, 5th Avenue Partners. Such a sum would buy access to private planes, yachts and protection, allowing Brown to live in luxury and move between islands at will. The disclosures are the latest developments in a saga which has dogged the Lib Dems since they accepted the donation six years ago. It has led to demands from Brown's victims – who are still owed millions of pounds – for the cash to be returned by Nick Clegg.

Sir Alistair Graham, who was chairman of the Committee on Standards in Public Life when the £2.4m donation was received, suggested that the party's grandees could devise a strategy to make incremental payments to Brown's victims.

"This does provide a problem for the Liberal Democrats. The public will have many questions about their integrity in these matters particularly when some individuals are suffering as a result of the fraud perpetrated by this man.

"While we have to be realistic – the Lib Dems are a members of a relatively small party and do not have this kind of money to hand – it may be time for the party's leaders to make a long term plan to return this money," he said.

City of London police also believe he has been helped by at least one accomplice to go on the run. Detective Superintendent Bob Wishart, who is leading the review of the case, said that his whereabouts remain a top priority for the force: "We believe that he has fled the country, likely with assistance, and has considerable assets at his disposal, but that does not diminish our desire to see him face justice. Our aim remains for him to be returned to the UK to serve his prison sentence. We urge anyone with information to come forward."

Brown, the son of a whisky executive, appeared to come from nowhere when he approached the Lib Dems with a donation in early 2005. He was not registered to vote, lived in Mallorca and had never been a party member, but offered a huge sum to the then leader Charles Kennedy weeks before a general election campaign. The money was spent on advertising.

Four months after the party celebrated its most successful election since the war, Brown was arrested over suspicions that he was engaged in a high yield fraud. A court subsequently concluded that he had donated money to the Lib Dems to give himself an air of respectability while defrauding investors.

Brown was accused of stealing nearly £50m from four investors, including £11m from Martin Edwards, the former Manchester United chairman.

He convinced potential "clients" that he was a highly educated member of the landed gentry who would use US security services to vet potential investors. The reality was that he had few qualifications, had run a string of failed businesses and was subject to an outstanding warrant for cheque fraud in Florida.

He had spent the proceeds of his crimes on a life of luxury. He owned a jet – in which he flew Kennedy and other Lib Dem officials across the UK – a Bentley and a yacht, and rented an office in Mayfair. His sprawling Mallorcan mansion was close to a holiday home of the model Claudia Schiffer and he would entertain friends with an outside cinema.

Awaiting trial in June 2008, Brown jumped bail while living in Hampstead. Weeks earlier, he changed his name to Campbell-Brown on bank documents and the electoral roll. Neighbours said he had grown a full grey beard and applied for credit cards using his new name before going on the run. Since then, private detectives working for two of Brown's creditors have claimed that someone using the name Campbell-Brown has used credit cards on at least two Caribbean islands.

Sources close to the investigators have confirmed that Brown is still believed to be moving between islands in the Caribbean, but has adopted another UK identity.

City of London police launched their review of Brown's case last year. About £36m of stolen money had passed through the bank accounts of 5th Avenue, but so far only £16m has been recovered or accounted for. Officers believe he has access to "substantial" funds.

Brown has also been placed on Crimewatch's Most Wanted list where he is described as 1.9 metres (6ft 2in), heavily built with a round face and greying hair.

Detectives have approached a number of his former associates for information.

A newly-ennobled Lib Dem befriended by Brown has been assisting police, a party source has confirmed. Lord Strasburger, a retired Bath-based businessman who was elevated to the House of Lords in January, put up £250,000 for Brown's bail money as well as paying for most of Brown's legal fees.

With his wife Evelyn, he has donated more than £760,000 to the Lib Dems over the past six years. Strasburger said through a party spokesman that he has co-operated fully with the police. "Without going into specific figures, it is true that I lost out financially in the Michael Brown case," he added.

An inquiry by the Electoral Commission, the political donations watchdog, cleared the Lib Dems of any wrongdoing in 2009 on the grounds that Brown's company was a permissible donor when it gave the donation, and that the party had acted in good faith. But Brown's victims continue to call for the party to return their stolen cash.

Robert Mann, the US tax attorney whose $5m (£3m) investment with Brown provided a proportion of the Lib Dem donation, said he hoped the renewed police investigation would shame Clegg in to authorising repayments.

"I hope that everyone involved in this travesty will ultimately do the honourable and right thing," he said.