The current procedure, which began on Saturday, involves a cap placed over the leak that gathers the oil, allowing it to be siphoned up via a pipe to a container ship.

'Massive clean-up'

John Terrett, Al Jazeera's correspondent in Louisiana, said: "The deadline comes as BP shares slump on world markets.

in depth

"They went down five per cent on Tuesday and down three per cent today based on fears that the dividend normally paid to shareholders will be diverted to pay for the massive clean-up operation in Louisiana and elsewhere in the Gulf coast.

"Meanwhile, oil has been drizzling onto the beaches in different parts of this region's beaches, including Mississippi, Alabama and Florida.

"The very worry is that the current will swirl the oil through the Florida Keys around up the east coast to New York."

Coast Guard Admiral Thad Allen said on Wednesday that BP had captured a little more than 15,000 barrels of oil on Tuesday with its "top hat" containment cap system.

Allen said BP was working to increase processing capacity of a drillship and a service rig at the water's surface to 28,000 barrels a day to handle the load as the company ramps up the collection rate.

Recovery interruptions

Tuesday's letter, which was addressed to Doug Suttles, BP's chief operating officer, highlighted concerns about the capacity of the ship processing the oil and fears that operations could be derailed during the ongoing hurricane season.

"The system[s] established must have appropriate redundancies to maintain complete collection rates in the event that operational problems are encountered in any part of the system," it said.

"For example, if multiple oil recovery vessels are employed for collection/recovery efforts, redundancies must ensure that the failure of a vessel[s] does not reduce the capacity of the system for continuous recovery of oil."

"There should be no interruptions of the recovery effort while awaiting another recovery vessel to arrive on scene.

"Further, plans and processes must be put into place to ensure that, in the event that a hurricane or other severe weather causes recovery vessels to go off station, those vessels [or alternate vessels] can be brought back on station as quickly as possible after the storm passes and that collection efforts can resume without delay."

The letter is another sign of the government stepping up the pressure on BP.

It comes as Barack Obama, the US president, faces growing political fallout over his administration's response to the crisis, which began after an offshore drilling rig exploded on April 20, killing 11 workers.

By the most conservative estimate, 26 million litres of crude have spilled into the Gulf, fouling Louisiana's marshes, coating birds and other wildlife, and curtailing fishing.

US officials say the actual tally could be much higher.