Two international money-laundering investigations are examining a controversial deal in which a Surrey mansion, a wedding gift from the Queen, was sold for £3 million more than its asking price.

Swiss and Italian prosecutors are probing how the Duke of York’s Surrey mansion, Sunninghill Park, a wedding present from the Queen to her second son, was bought by an offshore trust belonging to a billionaire Kazakh tycoon, reported the

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They are investigating whether bribes allegedly paid by Italian companies to the Kazakh regime could have been used to fund the Duke’s marital home and also investigating whether "a network of personal and business relationships" was allegedly used for "international corruption".

Despite having languished on the market for more than five years with a £12 million price tag, Sunninghill was sold for £15 million in 2007.

The estate was bought by an offshore trust belonging to Timur Kulibayev, the billionaire son-in-law of Kazakh president Nursultan Nazarbayev, who met Prince Andrew several times during the royal’s role as the Government’s trade ambassador.

There was speculation at the time that it was a favour to the Duke because of his close relationship with Kazakhstan – rumours fuelled by the fact that the 12-bedroom mansion has stood empty since the sale and is apparently derelict.

Socialite Goga Ashkenazi, who brokered the deal, said the Duke asked Kulibayev for £15 million, it was agreed on and the deal was ‘like any property deal between friends’.

The

that the two investigations centre on the activities of Hong Kong-registered 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.

£6 million for the purchase of the house allegedly came from Enviro, an oil and gas company that charges multi-million-pound fees to businesses seeking energy contracts in Kazakhstan.

The Sunday Telegraph claimed that in a letter from Italian prosecutor Fabio de Pascquale to his Swiss counterpart, he said he was aware of allegations surrounding the Sunninghill sale involving Enviro Pacific Investments, which he described as linked to ‘the summit of the Kazakh government’.

An indictment lodged with a court in Milan earlier this month accused several Italian companies of paying bribes to the Kazakh regime, including Kulibayev, who is alleged to have received £12.8 million.

Swiss prosecutors launched an investigation in 2010 into allegations that Kulibayev used Swiss banks to launder the proceeds of fraud. Claims he vehemently denies along with denying claims that he misappropriated millions of pounds in his former job, running Kazakhstan’s state-owned oil industry between 2000 and 2005.

Milan prosecutor de Pasquale is due to ask a judge for account details to be disclosed so that key financial transactions from Italy to Kazakhstan can be tracked.

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.”

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.”