An oligarch with close links to the Kremlin may have secretly taken ownership of a company responsible for anti-money laundering checks on Russian cash, the Paradise Papers show.
Isle of Man-based Bridgewaters should conduct independent due diligence tests on firms it administers, including dozens linked to Alisher Usmanov.
If he owns or controls the firm it would be a clear conflict of interest.
He and Bridgewaters strongly deny it is owned or controlled by the Russian.
Bridgewaters has been involved in major deals involving Russian cash, including the purchase of significant stakes in Facebook.
The Uzbek-born billionaire is a major client of Bridgewaters and if he does own or control the company it could be carrying out financial checks on Mr Usmanov's own offshore business activities.
Some of Mr Usmanov's $15.8bn (£9.97bn) fortune is in a private trust company called Bordeaux Limited.
Bordeaux used to be managed by offshore law company Appleby, the source of much of the Paradise Papers leaks, but in 2011 the private trust's management was transferred to Bridgewaters.
An internal Appleby email from 2011 uncovered in the Paradise Papers about the transfer of Mr Usmanov's Bordeaux private trust states that "the client has now bought a trust company in the Isle of Man (called Bridgewaters)".
Company filings show Mr Usmanov's business associate, Matthias Bolliger, was a director of Bridgewaters from 2011 to 2015.
And a leaked 2015 document from the Paradise Papers lists Mr Bolliger as "an ultimate beneficial official owner" of the company.
Author and taxation expert Nicholas Shaxson told BBC Panorama: "These companies are supposed to have due diligence requirements to look at what their clients are doing and make sure that this is not criminal money.
"If the billionaire controls the company that's supposed to be doing the due diligence on his own stuff it's a massive conflict of interest."
Lawyers acting for Bridgewaters told BBC Panorama that their client was not secretly owned or controlled by any individual.
The lawyers, who also act for Mr Bolliger, said he had never controlled Bridgewaters on behalf of any other individual and would be unable to do so as Bridgewaters was a regulated Isle of Man financial services company with a board of directors.
Chief Minister for the Isle of Man Howard Quayle told Panorama he could not comment on individual cases but if there was evidence Mr Usmanov was approving his own deals "we will have it thoroughly investigated and if there has been any wrongdoing whatsoever, we will take the relevant action. Whether that's a prosecution or withdrawal of right to operate on the Isle of Man".
Other revelations in the Paradise Papers have questioned whether Mr Usmanov may be secretly linked with both Arsenal and Everton in a way which if true could be a breach of Premier League rules.
Mr Usmanov's business partner Farhad Moshiri bought a 49.9% stake in Everton.
The Everton deal was administered by Bridgewaters.
The company that owns Everton, Blue Heaven Holdings, is registered at Bridgewaters.
One of its directors is an employee of Bridgewaters and the other is an employee of Mr Usmanov.
Panorama has also seen a list of dozens of British Virgin Islands (BVI) companies linked to Mr Usmanov that are managed by Bridgewaters.
Under anti-money laundering rules the true, or beneficial, owners of companies should be disclosed.
However, the ownership details of most of the companies on the list remain hidden.
The Panamanian law firm, Mossack Fonseca, often known as Mossfon and which was the source of last year's Panama Papers leaks, acted as an agent for Bridgewaters and was clearly frustrated by the lack of information it was getting about Mr Usmanov's companies.
One Mossfon email says: "The answers on the incorporation forms are ambiguous normally, given in a manner that can be interpreted in various ways. They are trying to give information without giving it if you know what I mean... now they have raised a bit of suspicion."
After Mossfon raised questions about compliance, Bridgewaters transferred most of its BVI companies to another financial services company, SHRM Trustees, in February 2015.
The papers are a huge batch of leaked documents mostly from offshore law firm Appleby, along with corporate registries in 19 tax jurisdictions, which reveal the financial dealings of politicians, celebrities, corporate giants and business leaders.
The 13.4 million records were passed to German newspaper Süddeutsche Zeitung and then shared with the International Consortium of Investigative Journalists (ICIJ). Panorama has led research for the BBC as part of a global investigation involving nearly 100 other media organisations, including the Guardian, in 67 countries. The BBC does not know the identity of the source.
Paradise Papers: Full coverage; follow reaction on Twitter using #ParadisePapers; in the BBC News app, follow the tag "Paradise Papers"
Watch Panorama on the BBC iPlayer (UK viewers only)