A $6 billion (3.8 billion pound) lawsuit between two of Russia's best-known tycoons ended in London on Thursday after a four-month High Court hearing that provided a sometimes deeply unflattering insight into the ruthless business world of post-Soviet Russia.

Judge Elizabeth Gloster reserved her judgement in the case, in which former Kremlin insider Boris Berezovsky claimed he was extorted into handing the crown jewel of his business empire to Roman Abramovich, the billionaire owner of Chelsea football club.

Her ruling, expected at the end of March or in early April, will bring the curtain down on one of the biggest private lawsuits in British history, with legal costs alone reported to be as high as 100 million pounds.

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

The titanic legal battle has provided rich media pickings in Britain ever since a tussle between the two tycoons and their retinues of bodyguards in a Hermes luxury boutique in London in 2007, when Berezovsky served Abramovich with a writ.

Hearings provided details of the extravagant lifestyles enjoyed by Russia's super-rich, with helicopter flights to luxury skiing resorts in the Alps, Caribbean cruises, large English country estates and French chateaux.

The two key witnesses traded insults during the 43 days of hearings, Berezovsky dismissing his one-time ally as not smart while Abramovich described his former mentor as something of a megalomaniac.

MURKY BUSINESS WORLD

The case threw a powerful spotlight on the conduct of business 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.

A Kremlin insider in the 1990s under former President Boris Yeltsin, Berezovsky left Russia in late 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 his political patronage and protection from criminal gangs, 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.

The trial has been followed closely by Russia watchers from London and Moscow for new clues into Russian business and politics under Putin, now prime minister but expected to become president again this year.

(Editing by Jon Boyle)