A buying agent who had been approached about the flat told CNBC that it had been repossessed.
Several sources in the London property world said the one-bedroom apartment, in a building where the most expensive flat famously went for around £100 million ($158 million), was once owned by Ray Grehan, an Irish property entrepreneur who went bankrupt in the U.K. recently.
Grehan was previously embroiled in a dispute with Ireland's National Asset Management Agency over whether an apartment in One Hyde Park, which he bought in 2007 for £3.7 million, still counted as part of his assets after it was sold to a company linked to his family trust. The mortgage on the property is with Barclays, who may have appointed the agents.
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Others in the property world claimed the flat was owned by Suleyman Kerimov, the Dagestani potash billionaire, although one source pointed out the flat would probably not be expensive enough for him. Kerimov, formerly a close ally of Russia's President Vladimir Putin, has an estimated net worth of $7.1 billion, according to Forbes.
Neither Kerimov nor Grehan returned calls from CNBC yesterday.
The development, put up by developers Candy & Candy, designed by architect Lord Rogers and backed by Qatari money, has become well known for being mainly owned by wealthy out-of-towners.
Out of 76 apartments at One Hyde Park for which Land Registry records are available, only 12 are registered to people.The rest are officially owned by corporations, only three of which are based in London, according to Vanity Fair.
The development has become an emblem of the excesses of London's luxury property market—and the fact that one flat has been repossessed is quickly becoming a symbol of the pitfalls facing even the wealthiest post-credit crisis.
—By CNBC's Catherine Boyle. Follow her on Twitter @cboylecnbc.