The Russian half of the Russian-British TNK-BP joint venture is threatening to file a law suit for up to $10m against its parent company BP, saying BP breached its shareholder agreement for going straight to state oil company OAO Rosneft rather than through TNK-BP, sources said.

Rosneft and BP signed a share swap deal in January which agreed the joint development of the Russian Arctic shelf and entitled Rosneft to 5 percent in BP in exchange for 9 percent of its own shares. The deal however was blocked by AAR, the company representing the interests of four billionaires in TNK-BP, because it felt the contract with Rosneft had broken their own agreement.

TNK-BP subsequently claims losses because of BP’s actions, saying it has lost an opportunity to explore within the Russsian Arctic and will therefore be seeking damages.

The $18bn deal between BP and Rosneft was left in limbo however, after its key supporter Igor Sechin stepped down as Rosneft’s chairman on Monday. Sechin, who is also the deputy prime minister of Russia, resigned from his position after President Dmitry Medvedev ordered the removal of ministers from the board of state companies.