The deal which saw Prince Andrew's marital home sold for £3 million more than its asking price is being examined by two international money-laundering investigations.
Prosecutors in Italy and Switzerland are looking into how the purchase of the Sunninghill estate was put together by a group of oil executives.
The inquiries are focusing on "a network of personal and business relationships" allegedly used for "international corruption".
Under the spotlight is what was decided when a group of wealthy Kazakh oil executives and Prince Andrew's close friend Goga Ashkenazi met at a Thai beach resort, when the deal to buy the Prince's home was discussed.
The sale of the Sunninghill estate, a wedding gift from the Queen to the Prince and his then wife, the Duchess of York, has already raised concerns after it was disclosed the buyer was an offshore trust belonging to Timur Kulibayev.
Mr Kulibayev is the billionaire son-in-law of the Kazakh president, Nursultan Nazarbayev, with whom Prince Andrew had a number of meeting as part of his former role as the Government's trade ambassador.
The two investigations centre on the activities of Enviro Pacific Investments, a company based in the British Virgin Islands, a tax haven which offers secrecy to companies and individuals who conduct their affairs there.
Enviro Pacific Investments is an oil and gas firm working in Kazakhstan, which charges multi-million pound fees to other firms seeking energy contracts in the country.
In the course of the meeting at the resort the trust is understood to have agreed to put up at least £6 million towards for the purchase of the house.
Sunninghill, made up of a house once nicknamed derisively Southyork for its resemblance to an American ranch home, and its five acres of grounds were purchased by the trust in June 2007, having been on sale for more than five years at a price of £12 million.
Despite the £15 million price tag, the house has been left empty ever since and is now apparently derelict. The Duke moved to Royal Lodge in Windsor, which remains his home.
The meeting at the Thai resort of Amanpuri, Phuket, between January 31 and February 5 2007 involved 18 people.
Among those at the meeting were the Duke of York's close friend Miss Ashkenazi, a socialite and businesswoman, and Arvind Tiku, a wealthy Indian who is Mr Kulibayev's business partner.
Other guests included Marianna Belchanskaya, a Russian lawyer, and Kairat Boranbayev chairman of the Russian-Kazak Oil company KazRosGaz, which is jointly owned by Kazakhstan and the Kremlin.
They were also joined by a mystery man, who stayed in Miss Ashkenazi's private villa, who used the alias "John Smith" on the resort's reservation record.
The Italian authorities are interested in the trust because of allegations of corrupt payments made by Italian firms to politicians in Kazazkhstan.
On Tuesday Milan prosecutor Fabio De Pasquale is due to ask a judge for account details to be disclosed so financial transactions from Italy's ENI group to Kazakhstan can be tracked.
An indictment lodged with the court earlier this month accused several Italian firms of paying bribes to leading members of Kazakh President Nazarbayev's regime, including Mr Kulibayev, who is alleged to have received £12.8 million.
Italian prosecutors are looking into the possibility that the bribe may have been used to buy Sunninghill Park, although there is no firm proof as yet and that is why the request for bank details to be disclosed is being made to a Milan judge.
Mr De Pasquale said: "The investigation is still in the early stages we are making a request for information on the 29th May and then we will see what happens. At this moment in time I cannot say anything else."
In letters to Lucie Wellig, his counterpart in the Swiss prosecutor's office, written in February last year and March this year, he asked for her assistance in gathering evidence.
In the letters, obtained by the Sunday Telegraph, he said he was aware of allegations surrounding the Sunninghill sale involving the firm Enviro Pacific Investments, which he described as linked to people "at the summit of the Kazakh government".
Details of the meeting in Thailand obtained by the Sunday Telegraph show that Mr Tiku used his assistant to arrange for $140,000 (£89,370) to be paid in advance to cover the Kazak team's expenses during their stay, plus an additional $40,000 (£25,550) in cash in an email headed "hotel and stay expenses - Kazak team - Phuket -Enviro Pacific Investments Ltd",
On other hotel bills and receipts, the party is referred to as the Ocean Oil Group, part Mr Tiku and Mr Kulibayev's business empire through which they control oil and gas fields in Kazakhstan.
Miss Ashkenazi stayed in Villa number 7, a £2,000-a-night pavilion, which she shared with "John Smith" for the five night trip between January 31 and February 5 2007.
Mr Tiku paid for three $950 dollar a night rooms and four Thai-styled Pavilions located away from the hotel "interspersed throughout a coconut plantation".
Guests spent £30,000 on food and drink, and a further £3,500 on a fireworks display and £800 on a jazz band to celebrate the birthday of one of the guests, who also ordered several Cuban Cohiba mini cigars on their tab costing £17 each.
Ten days after the Thai trip Bor Investments, the secretive offshore trust which owns Miss Ashenazi's £27.5 million home in Holland Park, west London, was set up.
Prince Andrew's close friendship with Miss Ashkenazi had blossomed during 2007 and he introduced her to the Queen at Lady's Day at Royal Ascot in June that year, when she had lunch in the Royal Box.
Last week she failed to answer questions sent to her via her London office. Other guests who were in Phuket also failed to respond to requests for comment. Sources close to the Prince said he was not present at the meeting.
Oxford University educated Ms Ashkenazi is chief executive of MunaiGaz Engineering Group, a multi-million-pound oil and gas company with operations in Kazakhstan, Russia and Ukraine. She has previously denied rumours of an affair with Prince Andrew as "such nonsense".
Ms Ashkenazi has said: "Andrew has been a very good friend of mine since we met at a dinner party in 2001. We have many common friends."
It is also unclear where Mr Kulibayev, who is married to President Nazabayev's daughter, was during the Thai trip. Miss Ashkenazi is rumoured to be his long time mistress and he is named as the father on the birth certificate of one of her two children.
Ms Ashkenazi has also dismissed the controversy over the purchase of Sunninghill for £15 million after it had languished on the market for more than five years and was valued at £12 million.
"It was a deal struck between friends and the asking price was paid,' she has said.
'I can't understand why there has been such a fuss. There was nothing corrupt about it. I think the people negotiating for Timur were not aware they were paying more money. I suspect they were instructed not to haggle with the Royal Family.'
Last night a Buckingham Palace spokesman, referring to the Sunninghill sale, said: "This was a private sale between two trusts. There was never any impropriety on the part of The Duke of York, any suggestions of which are false."
