BP has come under further pressure after one of its partners said the oil giant should foot the entire bill for the damage caused by the Gulf of Mexico spill.

Anadarko Petroleum, which owns a quarter of the ruptured Deepwater Horizon well, refused to accept any blame for the explosion that killed 11 workers and led to the US's worst environmental disaster.

The company's chairman and chief executive, Jim Hackett, said in a statement BP's actions probably amounted to "gross negligence or wilful misconduct".

BP's chief executive Tony Hayward, who was grilled about the disaster by Congress for seven hours on Thursday, said he "strongly disagreed" with the allegation and expected the firm's partners to "live up to their obligations".

The spat between the two companies came as industry experts warned that the out-of-control well will go on spewing oil into the Gulf of Mexico for the next two years or more if all attempts to contain or plug the gusher fail.

The estimates, based on new figures supplied by Hayward, suggest the potential environmental and economic devastation would far outstrip the damage done so far by the ruptured well, which has been spewing for 60 days.

Hayward told a Congressional committee on Thursday that the reservoir still held 50m barrels, providing fresh urgency to efforts to contain the oil, or seal off the gusher completely with a relief well.

Using the government's present flow estimates of up to 60,000 barrels a day, BP's well could go on gushing for two to four years, unless it is stopped.

BP and the US government say they are containing a rising share of the oil from the well, and hope to plug the gusher completely by August, when two relief wells will be complete. BP said today the relief wells were within 60 metres of the ruptured well.

The consequences of failure are enormous.

"If it went on uncontrolled, it could certainly leak for two years and certainly longer than that," said Philip Johnson, a professor of petroleum engineering at the University of Alabama.

But he said the rate of the leak would fall sharply once the natural gas in the reservoir is exhausted. "That is the driving force," he said. "As soon as that is gone, it won't leak at any serious rate."

Yesterday, BP's chairman Carl-Henric Svanberg said Hayward had been relieved of day-to-day control of the spill and that the company's managing director Bob Dudley would take over.

Svanberg admitted the oil rig explosion was a "tragic one" which "should never have happened".

He denied claims by the Russian president Dmitry Medvedev that BP faced possible "annihilation" over the spill.

"I think we have to put everything in perspective, of course this is a huge thing, it is a huge setback for BP [but] the company is strong, the company has strong underlying performance, strong cash flow, strong operations," Svanberg said.

Hayward has pledged that BP will foot the entire clean-up bill but insisted it is "too early to say" what caused the massive spill.