In 1999, somewhere between Moscow and the Mediterranean, a quiet paperwork operation began that would eventually touch billions of pounds. According to a new analysis of leaked corporate records known as the Cyprus Confidential files, aides to the Russian billionaire Roman Abramovich spent more than a decade reorganising his offshore business empire using documents that were, it appears, dated years after the events they claimed to record. The fortune in question traces back to Sibneft, the Siberian oil company Abramovich acquired during Russia's turbulent post-Soviet privatisation of the 1990s. The files, more than 500,000 of them, were first obtained by the International Consortium of Investigative Journalists and examined by the Guardian, the Bureau of Investigative Journalism and the BBC.

They point to Eugene Tenenbaum, Abramovich's longtime London-based aide, as a central figure in the alleged backdating, and suggest he may have given misleading testimony during a UK High Court case. Staff at Deloitte's Cyprus office, a supposedly independent accountancy firm, are also implicated in drafting paperwork that disguised when transactions actually occurred. What the documents expose is not a single lapse but a system: one designed, apparently, to make transfers across tax havens look older, and therefore safer from scrutiny, than they really were. Lawyers for both Abramovich and Tenenbaum deny any wrongdoing, insisting they acted on professional advice from Deloitte Cyprus, which says it followed all applicable laws and can no longer verify old files given how much time has passed.