British oil giant BP is desperately working on a new way of stopping its Gulf of Mexico oil leak today after its "top kill" method failed.
In the firm's latest setback, BP said the operation to block the hole with mud and other material had not worked.
"We have not been able to stop the flow," chief operating officer Doug Suttles said last night. "We have made the decision to move on to the next option."
Since the middle of last week, hopes had been growing that the top kill method was beginning to stem the flow of oil after the US Coast Guard said the well had stabilised. But the company said several days would be needed to gauge success.
The spill is the worst in US history and has dumped between 18 and 40 million gallons into the Gulf, according to government estimates.
BP said it was already preparing for the next attempt to stop the leak - using a robot submarines to cut off the damaged riser from which the oil is leaking, and then trying to cap it with a containment valve.
The new attempt would take four days to complete.
"We're confident the job will work but obviously we can't guarantee success," Mr Suttles said of the new plan.
Oil has been spewing from the well since the Deepwater Horizon rig exploded and sank on April 20, killing 11 workers and sparking an environmental disaster.
Other attempts to stop the leak have included placing a gigantic box over the leak and a tube inserted to siphon the oil away. The box failed after ice-like crystals clogged it, while the tube was removed to make way for the top kill after it sucked up more than 900,000 gallons of oil.
On Friday, US President Barack Obama toured oil-hit areas in Louisiana, saying he would "do whatever it takes" to help those affected.
He said additional manpower would lay more booms, clean beaches and monitor stricken wildlife.
Around 20,000 people have already been deployed to contain and clean up the spill.
President Obama also ordered BP to pay doctors' fees for any "ill-effects" caused by the Gulf of Mexico oil crisis.
The company previously said it would pay out economic injury claims. Around 26,000 claims had been filed so far and 11,650 payments have already been made.
The group has also received more than 96,000 calls to its help lines.
BP said that, as well as the top kill operation, it was continuing to drill two relief wells at around 13,000ft below the seabed in an attempt to cut off the flow of oil, but these are expected to take three months to complete.
America's previous worst oil spill was the 1989 Exxon Valdez disaster, in which a tanker ran aground in Alaska, spilling nearly 11 million gallons.
BP said its efforts to stop crude from gushing into the sea had cost nearly one billion dollars (£688 million).