BP says oil has stopped leaking into the Gulf of Mexico from the Deepwater Horizon rig for the first time since April.

The oil well has been sealed with a cap, which engineers are now monitoring to see if it holds.

They will watch pressure readings to see if the cap is working. High pressure is better, because it means the oil is staying within the cap and not escaping through an unknown leak. Low pressure means there could be another leak.

BP executive Kent Wells said the oil stopped at 7.25pm UK time.

He added: "It is very good to see no oil go into the Gulf of Mexico."

The cap, installed on Monday, is a crucial step toward a four-vessel oil capture system that is hurricane-ready and can collect up to 80,000 barrels a day. The first of two relief wells is expected to intercept and plug the leak by mid-August.

It represents the best hope yet of stopping the oil from leaking into the sea since the 20 April Deepwater Horizon explosion that killed 11 people.

A BP spokeswoman said: "Information gathered during the test will be reviewed with the relevant government agencies, including the federal science team, to determine next steps.

"The sealing cap system never before has been deployed at these depths or under these conditions, and its efficiency and ability to contain the oil and gas cannot be assured."

Tonight President Barack Obama said news was a "positive sign," but reminded reporters that the latest effort was still in a testing phase.

The company has come under increasing pressure in the United States amid reports that it lobbied the British government for the release of the Lockerbie bomber in what is being called an oil-for-terrorist deal.

Hillary Clinton, the US secretary of state, pledged to consider Congressional demands for an investigation into the charges that BP pushed for the release of Abdel Basset al-Megrahi to help it clinch lucrative drilling contracts off the coast of Libya.

Tests of the latest containment attempt began with BP shutting off pipes that were funnelling some of the oil to ships on the surface, meaning the full force of the gusher went up into the cap. Deep-sea robots began slowly closing three openings that let oil pass through. Ultimately the flow of crude will be blocked entirely.

The cap was lowered on to the well on Monday but not before the White House had added to the pressure on BP, putting a 24-hour delay on the deployment of the cap so engineers could perform additional safety reviews.

The US government had been concerned about whether the buildup of pressure from the gushing oil could rupture the walls of the well and make the leak worse.