Boris Berezovksy's $6 billion (3.9 billion pounds) lawsuit against rival Russian tycoon is doomed to fail because key witnesses are dead and no written records exist of crucial agreements at the heart of the case, London's Commercial Court was told on Wednesday.

The trial, expected to end on Friday with a judgment due weeks later, has pitted former Kremlin insider Berezovsky against one-time ally Roman Abramovich, the billionaire owner of Chelsea soccer club.

Berezovsky, 65, says Abramovich intimidated him into selling his stake in oil firm Sibneft - the crown jewel of his business empire - at a knockdown price. Abramovich, 45, denies Berezovsky ever had an interest in the firm.

The burden of proof is on Mr Berezovsky to establish his claims, said Ali Malek, a lawyer representing an opponent of Berezovsky in a separate case that partly overlaps with the Abramovich lawsuit.

There were insuperable difficulties in the case for Berezovsky as the agreements were all oral.

The court would normally expect, in view of their importance, that the alleged agreements be recorded in writing or certainly evidenced in writing by contemporaneous documents and here they were not, Malek said.

The court is being asked to make findings of fact in 2012 in relation to agreements some 16 to 17 years ago... Important witnesses are dead, he added.

The titanic legal battle between two of the wealthiest and most powerful men to emerge from post-Soviet Russia has provided rich media pickings ever since a tussle between the two tycoons and their retinues of bodyguards in a Hermes luxury boutique in London, when Berezovsky served Abramovich with a writ.

RUSSIA'S ROARING 90s

And it has thrown a powerful spotlight on the shady business dealings in Russia in the decade following the collapse of the Soviet Union, when opaque deals to sell off state assets turned a handful of insiders into the owners of multi-billion dollar natural resources companies.

It is this kind of unusual background and context in Russia that further complicates the case, Malek said.

Spoken agreements between big businessmen in Russia were not uncommon in the 1990s because they were reluctant to disclose their assets.

Judge Elizabeth Gloster is also being asked to rule whether Abramovich's payments to Berezovsky were for his political patronage and protection from criminal gangs - an informal arrangement known in Russian slang as krysha - or dividends from Sibneft.

During the morning part of Wednesday's hearing, Berezovsky appeared cheerful, sitting next to his younger girlfriend, his large legal team and bodyguards. But by the afternoon, he was yawning loudly.

Abramovich did not attend Wednesday's hearing.

A close ally of former Russian President Boris Yeltsin during the 1990s, Berezovsky left Russia in 2000 after falling out with Yeltsin's hand-picked successor Vladimir Putin.

He says he gave up his Sibneft stake because he feared that if he refused, Abramovich would ensure Putin had the shares expropriated.

Abramovich says he paid Berezovsky $2 billion for krysha, but not as dividends from Sibneft because Berezovsky was never an owner. Abramovich has since sold Sibneft to the Russian state natural gas monopoly Gazprom.

Berezovsky also accuses Abramovich of selling, without permission, his shares in aluminium producer RUSAL.

(Editing by Jon Boyle)