July 6 (Bloomberg) -- AAR, the Russian partner in the TNK-BP oil venture in which BP Plc holds 50 percent, said today it is being sued for 8.5 billion rubles ($362 million) by BP, in an escalation of the tensions between the parties.

AAR learned on July 3 that BP submitted the claim for damages against it in the High Court of London for an alleged breach by AAR of a tax deed of covenant dated Aug. 29, 2003, AAR said in an e-mailed statement today.

``While the full details of the legal claim have not yet been submitted by BP, AAR considers the claim unjustified and grossly inflated,'' AAR said in the statement. ``AAR will vigorously defend against it.'' BP said in response to the statement that the claim is ``perfectly justified.''

BP, whose 50 percent stake in TNK-BP accounts for a quarter of the company's total output and a fifth of proved reserves, is struggling to maintain control of the company. Billionaires Mikhail Fridman, German Khan, Viktor Vekselberg and Len Blavatnik, who control the other 50 percent and collectively form AAR, say London-based BP treats TNK-BP as a subsidiary and stunts its development as an independent company.

The back-tax claim predates the formation of the TNK-BP venture. AAR said today it has to date paid close to $500 million to BP in connection with the tax deed of covenant.

Exact Amounts

BP and AAR have been in discussions for the past 12 months over the exact amounts owed by each party to the other, Stan Polovets, chief executive officer of AAR, said in the statement. ``We are very surprised that instead of trying to resolve this matter through dialogue, BP has turned to British courts with a claim that is grossly inflated,'' he said.

``AAR considers the claim as another example of BP's bullying tactics and an attempt by BP to deflect the focus away from its mismanagement of TNK-BP,'' Polovets said, according to the statement. ``BP's lack of willingness to treat us as equal partners and constructively deal with the differences between the shareholders is one of the main reasons behind the current tensions,'' he said.

``It's a perfectly justified claim,'' BP spokesman Roddy Kennedy said in a telephone interview after AAR released its statement. ``It's got nothing to do with bullying. AAR owes BP this money and has done for some time, so shouldn't be surprised that we've claimed it,'' he said.

The suit has ``nothing to do with the current dispute'' over control of TNK-BP, Kennedy said.

Vekselberg has called an extraordinary board meeting for tomorrow to discuss removing CEO Robert Dudley from management.

U.K. Prime Minister Gordon Brown and Russian President Dmitry Medvedev are likely to discuss the dispute at the Group of Eight summit that starts tomorrow, an aide to Medvedev said on July 3. Both the Russian government and state-run energy producer OAO Gazprom have repeatedly denied being behind the tussle.

To contact the reporter on this story: Sebastian Alison in Moscow at Salison1@bloomberg.net

Last Updated: July 6, 2008 14:40 EDT