UPDATE 1-Russia's Nornickel co-owners reach conditional share deal

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LONDON, March 8 (Reuters) - Russian billionaire Roman Abramovich has struck a deal that would allow him to sell a 4 percent stake in miner Norilsk Nickel to fellow shareholders Vladimir Potanin and aluminum producer Rusal, pending the outcome of a court case, a lawyer for Rusal said.

Under the provisional arrangement, Abramovich's Cyprus-based company Crispian would be able to sell the shares to Potanin and Rusal in proportion to their existing holdings in Norilsk Nickel (Nornickel), the lawyer told a London court on Thursday.

However, any transfer of shares would be reversed if the court rules in favor of Rusal in a dispute.

Rusal is trying to stop Chelsea soccer club owner Abramovich from selling Nornickel shares, arguing that would violate a 2012 shareholder agreement.

Interros, which manages Potanin's assets, said it planned to complete its purchase of a 2 percent stake in Nornickel from Abramovich next week. Rusal did not immediately reply to requests for comment.

The ownership dispute is part of a long-running battle for control of Nornickel, a $31 billion company that competes with Brazil's Vale for the rank of the world's top nickel producer. It is also the world's largest palladium producer.

Potanin owns a 30.4 percent stake in Nornickel and Rusal, controlled by billionaire Oleg Deripaska, holds 27.8 percent. Abramovich and his partners hold a 6.3 percent stake.

Judge Christopher Butcher said the trial over the legality of the share sale by Abramovich would start on May 14 unless the parties appealed for a delay by April 13. (Reporting by Peter Hobson Additional reporting by Polina Devitt Writing by Peter Hobson and Polina Devitt Editing by David Goodman and Mark Potter)