Arkady Gaydamak is suing his former partner Lev Leviev over profits from a lucrative diamond mining venture in Angola.
The case, currently being held at the High Court in London, is expected to close early this week, with a judgement expected later the in year.
It is understood that Gaydamak, father of former Portsmouth Football Club owner, Alexandre Gaydamak, is using The PR Office on a project basis during the case.
The agency acknowledged to PRWeek it is representing Gaydamak around the litigation, but declined to comment further.
Gaydamak claims he and Leviev signed an agreement in 2001 to share profits from the diamond trading venture. He said the agreement was given to the Chief Rabbi of Russia, Rabbi Berel Lazar, for safekeeping, but has subsequently been lost or destroyed.
Leviev claims the document was 'a different agreement' to do with charitable contributions.
Gaydamak gave evidence during the case via video link from Israel as he is fighting extradition to France after a conviction for tax fraud.