Russia's migration service said it would not give a visa to TNK-BP Chief Executive Robert Dudley without a valid contract, a move that may help the Russian-connected co-owners oust the BP-backed executive.

The move is the latest in a long-running battle at TNK-BP, a 50-50 joint venture between BP and four oligarchs, over the firm's management and strategy. TNK-BP is Russia's second- biggest foreign investment and investors are following the fight closely.

"We cannot grant a visa to Mr Dudley before he presents a valid contract. The previous contract expired and we have no proof that it has been prolonged," Konstantin Poltoranin, a spokesman at the Federal Migration Service (FMS) told Reuters on Monday.

Dudley's contract expired last year but BP says it is still valid under Russia's Labour Code because it has never been officially terminated and therefore rolls over automatically.

The four Russian billionaires, who want to sack Dudley as part of their dispute with BP, say the contract has never been officially renewed and is therefore no longer valid.

A source close to TNK-BP dismissed as "nonsense" the idea that Dudley did not have a valid contract, saying a board resolution tabled by the Russian oligarchs to sack Dudley earlier this year in itself proved that they recognised his contact was valid.

Dudley has a valid Russian work permit but needs to present a valid work contract to get his visa renewed. His visa expired on July 19, but the migration service agreed to give him a transit visa, which is valid until July 27.

"He (Dudley) has been given 10 days to present a contract. We may extend his visa one more time, but without a valid contract, he cannot work in Russia," the FMS's Poltoranin said.

Dudley is also facing penalties from a judge after a labour inspector found some minor violations, the TNK-BP source said. These could include suspending him from his post for a period of up to three years.

"Obviously the pressure is mounting," the source said. "The likelihood is that somehow they will try to engineer Dudley's suspension this week so that the Migration Service can then say they don't need to consider his visa request and he has to leave."

"It's ridiculous but this is the reality."