A bird flies above oil seen on the surf and the shore in Gulf Shores, Ala., on June 12. (Lyle W. Ratliff/Reuters)

A U.S. congressman has disclosed an internal memo from BP that says the company's damaged oil well in the Gulf of Mexico could be gushing as much as 100,000 barrels of oil a day.

Representative Edward Markey, chair of the U.S. House's energy and environment subcommittee, released the document Sunday.

In the memo, BP says that in a worst-case scenario, the leak could gush 100,000 barrels a day if the blowout preventer, the wellhead and other equipment were removed from the sea floor, which was never done. It also assumes that rate if "we have incorrectly modelled the restrictions."

"So again right from the beginning BP was either lying or grossly incompetent," Markey told NBC's Meet the Press show. "First, they said it was only 1,000 barrels, then they said it was 5,000 barrels, now we're up to 100,000 barrels.

"They are the ones who should have known right from the beginning, and, either to limit their liability or because they were grossly incompetent, they delayed a full response to the magnitude of this disaster," he said.

The current U.S. government estimate of oil leaking from the blown-out well is 60,000 barrels a day, up from previous estimates of between 25,000 and 30,000 barrels.

In the early days of the leak, the company estimated 1,000 barrels of oil a day were spilling, and it resisted but eventually relented to government projections that put the flow at 5,000 barrels daily.

BP officials later told Congress the worst-case scenario could be 60,000 barrels a day.

BP spokesman Tony Odone said the documents disclosed Sunday were submitted to Congress before BP America president Lamar McKay testified in early May.

Biggest spill

The BP-leased Deepwater Horizon offshore rig exploded April 20, killing 11 workers and setting off the undersea gusher on the Gulf of Mexico sea floor.

BP has said it is "deeply sorry" for the disaster and is "absolutely responsible" for what has turned into the largest oil spill in U.S. history.

As late last week, the BP well had gushed between 250 million and 450 million gallons of oil into the Gulf, based on government estimates, destroying wildlife habitats, shuttering fisheries and coating beaches in black goo.

BP is trying to capture most of the leaking oil with a containment dome placed over the top of the wellhead to draw it to a barge on the water's surface, 1,500 metres above.

U.S. President Barack Obama said last week the containment efforts are expected to capture up to 90 per cent of the oil "in the coming days and weeks," but the problem would not be entirely fixed until BP finishes drilling a relief well, likely in August.

With files from The Associated Press