The report said the oligarch's offshore trusts were reorganised shortly before the UK and Europe imposed sanctions on him. UBS declined to comment; Barclays did not comment on the specific details of the case.

Russian oligarch and former owner of Chelsea Football Club Roman Abramovic was a major client of Barclays in Monaco and UBS in Zurich, The Guardian newspaper has reported. Abramovic’s offshore trusts were reorganised shortly before Europe and the UK imposed sanctions on him following Russia’s invasion of Ukraine, it said.

The leaks are part of what the UK newspaper referred to as the “Oligarch Files,” continuing a theme of such data hauls by media groups, such as the Panama Papers and Paradise Papers, in recent years.

The report said that before Russia invaded Ukraine, UBS and Barclays held at least $940 million of assets on behalf of Abramovich’s trusts and the companies which he ultimately owned. Just before military operations started, the 10 offshore trusts were rapidly rearranged to transfer beneficial ownership of a significant portion of Abramovich’s vast fortune to his children, the report said.

UBS declined to comment to WealthBriefing. A statement from Barclays said: “While Barclays does not comment on whether a person is a client or not, Barclays understands the importance of sanction regulation and Barclays’ role in preventing financial crime. Barclays takes its sanctions obligations very seriously and has robust systems, controls and processes in place; these include the prevention of circumvention of sanction rules. We are in regular communication with our regulators regarding sanction risks as well as our controls and oversight.”

On 10 March the UK government imposed sanctions on Abramovich for allegedly maintaining close relations with Vladimir Putin. The EU followed with its own sanctions on 15 March and Switzerland has – in an unprecedented move for the famously neutral state – endorsed the EU’s stance.

 Abramovich has denied the allegations and is appealing against the EU measures.

The Guardian said the files originate from the Cyprus-based offshore service provider MeritServus, which has managed Abramovich’s financial interests for more than 20 years. The firm administered 10 trusts established to benefit Abramovich and a sprawling network of offshore companies, it said.

On 8 March this news service reported on how UBS had detailed its Russia exposures. This publication has reflected on how Russia's invasion of Ukraine has put banks, and their relationships with Russian clients, into a harsh spotlight.