BP's 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, oil industry experts said today.

The estimates, based on new figures supplied by BP's chief executive, Tony Hayward, during seven contentious hours of testimony to Congress on Thursday, 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 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 administration 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 that the relief wells were within 60 metres of the ruptured well.

The consequences of failure are enormous.

"If it went 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 leak rate would fall sharply over time 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."

Hayward's statement to Congress represents the first definitive word so far from BP on the potential it saw in the Macondo well. BP exploration plans, submitted to the US government, had suggested production in the order of 15,000 barrels a day, a figure confirmed by Hayward to Congress.

But the 50m figure cited by Hayward took some industry insiders by surprise. There have been reports the reservoir held up to 500m barrels – the figure quoted by Hayward's questioner, Joe Barton, a Republican from Texas.

"I would assume that 500m barrels would be a more likely estimate," said Tadeusz Patzek, the chairman of the department of petroleum and geosystems engineering at the University of Texas at Austin. "I don't think you would be going after a 50mbarrel reservoir so quickly. This is just simply not enough oil to go after."

BP spokesmen said that Barton was referring to recoverable oil rather than the total size of the reservoir.

"We haven't made an assessment of the reserves as far as I know," said Toby Odone, a BP spokesman. "You start evaluating the reservoir once you complete the well. Obviously we didn't get to that point."

The figure was one of the few kernels of solid information offered up by Hayward, who was widely criticised for stonewalling and obfuscation in his appearance before Congress.

The performance did little for BP's financial position. Its shares fell back today after Moody's cut the company's credit rating to A2. Moody's said it was concerned about the escalating costs of the oil spill, and warned the $20bn (£13,5bn) compensation fund agreed earlier this week might not cover all of BP's liabilities.

"This assessment reflects a substantial upward revision of the estimated size of the leak, the continued failure to bring the leaking Macondo well under control, and the mounting costs and claims for damages," the ratings agency said.

"Uncertainty over the ultimate cost for massive litigation claims and other contingent liabilities will be an overhang on BP's creditworthiness that will persist for years to come," it added.

Ken Feinberg, who was appointed to oversee the $20bn claims fund, has promised to turn around claims within 60 days.

However, Hayward did get a vote of sympathy from John Hofmeister, a former president of Shell, who has himself testified to Congress on six occasions.

He noted Hayward was operating under the threat of criminal investigation, and said he would have taken a similar legalistic approach were he in the same situation.

"It's not possible for a CEO to know everything that is going on at every level of production," he told MSNBC television . "The Congressmen are very unfair to demand he know what happened at a meeting that took place four or five levels below him."