The Duke of Westminster, the property tycoon and the missing millions: Britain's richest man may be called as witness in a multimillion-pound fraud claim

The case relates to a house in London's exclusive Eaton Square that the duke once sold

Janna Bullock, who sold property in Moscow, New York and London, is a society figure in Manhattan

The Duke of Westminster, Britain's richest man, looks set to become caught in a multimillion-pound legal wrangle over a house he sold in exclusive Eaton Square, in Belgravia, west London.

It is claimed the duke's Grosvenor Estates empire sold the property for £3.8m in 2007 to a company thought to have been financed by one of Manhattan's most glamorous society figures, Janna Bullock. Three years later, with the London housing market roaring away once more, the house was sold on for a jaw-dropping £20.5m.

It is currently thought to be occupied by one of Azerbaijan's richest families, the Marandis, whose patriarch Javad Marandi is said to have two private jets and a passion for vintage wine.

It is not entirely clear how the Eaton Square house commanded such a huge mark-up between purchase and sale, although it is said that several leaseholds were bought from owners of flats within the building, and planning permission had been secured. It has been turned into a mansion, extending into the mews house behind, with an underground swimming pool and a gymnasium.

What is not in doubt, according to reports in the Russian state-sponsored news agency RIA Novosti, is that the proceeds from the 2010 sale have been frozen by a court order, which alleges that Ms Bullock and her associates are at the heart of a major fraud scandal in her native Russia, allegations she strenuously denies. From her home in New York, Ms Bullock is fighting all of the allegations with counter-litigation, claiming they are all part of a plot by business enemies to steal her assets from her.

The court order has reportedly been obtained by the country's powerful state-owned Gazprombank, which is basing its case on claims that Ms Bullock, who has been married four times, funded various purchases – including a luxury yacht – with the proceeds of a fraud she and a former husband perpetrated in a Moscow property scam. This allegedly involved government properties in which Gazprombank had invested.

Legal documents from Cyprus, seen by The Independent on Sunday, are claiming millions of dollars, in proceedings in which it is thought that the Duke of Westminster, Gerald Grosvenor, or his representatives, may be called as witnesses.

A spokeswoman for the Duke of Westminster and Grosvenor Estates, which are not accused of nor suggested to be involved in any wrongdoing, said the company had not yet been contacted to be a witness in the claim, adding that it would probably be a matter for the duke's staff: "I would very much doubt it would be his grace who would be called," she said.

If the duke's sales executives looked into her background back in 2007, Ms Bullock would have seemed like a typical New York society tycoon, with a position as a trustee of the Guggenheim Museum and a large philanthropic bequest. But in fact, she reportedly arrived in Manhattan almost penniless, after a humble upbringing in Russia and leaving her first husband behind.

She worked as a nanny, then is said to have got a job in a local deli before acting as a translator in a law firm specialising in Russian criminal cases. One day, she was dispatched by the firm to collect a well-connected Russian official, Alexey Kuznetsov, from JFK airport. At the time, she was reportedly married to her second husband, an American. Mr Kuznetsov had a Bulgarian wife, but the pair "became friendly right away", a source told the New York Post, and before long they were married.

According to reports, the marriage evolved into a business partnership, with Ms Bullock buying and developing tracts of land around Moscow, thanks to political cover from her new husband, who was then the finance minister for that part of the city. She reinvested the profits in luxury developments in New York, Paris and the Alpine resort of Courchevel, beloved of Russian billionaires, using her knack for buying run-down sites and turning them into luxury homes at a hefty profit.

She told The New York Times that her empire was at one stage worth $2bn. But now it all threatens to come crashing down. as Gazprombank claims it was swindled out of $20m.

According to the Russian state news agency, courts in Cyprus and the British Virgin Islands have frozen millions of dollars of the couple's assets around the world, which would include any profit made on the Belgravia sale.

A company called Hitnell, said to be Ms Bullock's, was used to buy the Eaton Square freehold.

Meanwhile, Ms Bullock has, according to the New York society pages, married again this year – this time to her art adviser Randall Brockett. "They had been working together for a while and fell in love," a friend told the New York Post.

Her ex-husband, Mr Kuznetsov, appears less happy. He reportedly fled Moscow for Europe, apparently in fear after a political ally was murdered in what looked like a contract killing. He was, however, detained by French police on an Interpol warrant in July near the Côte d'Azur, and now faces extradition to Russia on allegations that he stole hundreds of millions of dollars from the regional government that employed him.

Nobody responded to calls to the telephone number at Ms Bullock's website, or responded to emails. Her Manhattan lawyer, John Piskora, said: "Ms Bullock does not have any comment at this time." But when the scandal surrounding those famous former neighbours on Eaton Square, Nigella Lawson and Charles Saatchi, eventually dies down, Ms Bullock's colourful business and personal life will be giving Belgravia locals plenty to talk about in 2014.