BP drafts in more clean-up workers and closes Port Fourchon beach in Louisiana Reuters

BP today admitted that is capturing less oil from a ruptured well in the Gulf of Mexico than previously estimated, raising fresh questions over the success of its rescue operation.

The oil giant told the City this morning that the mile-long siphon tube inserted into the damaged well has been capturing an average of 2,010 barrels a day.

That is less than half the 5,000 barrels the company estimates is leaking into the sea every day following the Deepwater Horizon explosion. Last Thursday, BP told shareholders it was managing to siphon off around 3,000 barrels each day, while a BP spokesman in Houston told reporters that the siphon had actually been collecting 5,000 barrels of oil in a day.

The news comes after President Barack Obama warned that he would remove the company from efforts to seal the well if it does not act quickly enough to stop the leak, although he also acknowledged only BP and the oil industry know how to halt the spill.

Obama has now despatched two cabinet secretaries to the Gulf, keeping pressure on the company to plug the undersea oil leak which threatens to cause an environmental disaster. The oil has reached the delicate wetlands of Louisiana.

The spill, which has seen oil spewing into the ocean for more than a month after the rig explosion in April, has cost BP $760m (£526m) so far, up from the $625m estimate it issued on 18 May.

The US Coast Guard said more than 65 miles (110 km) of the Gulf coast has been hit by the oil spill, and less than half of it can be cleaned up quickly. The British oil giant said it continues to work on a "top kill" operation where heavy drilling fluids are injected into the well to stem the flow of oil and gas and ultimately kill the well.

"Most of the equipment is on site and preparations continue for this operation, with a view to deployment in a few days," BP said.

In today's statement, BP admitted that the oil captured by the "riser insertion tube tool", had fluctuated between 1,360 and 3,000 barrels of oil a day. It blamed the variable collection rate on "flow parameters and physical characteristics within the riser".

Shares in BP fell 1.6% this morning to 498p, their lowest level since last August. The company has now lost almost a quarter of its value since the Deepwater Horizon exploded.