BP engineers yesterday stopped oil flowing into the Gulf of Mexico for the first time since April as they shut all valves on a new cap placed on top of a fractured well-head, a BP official said.

"It is good to see no oil going into the Gulf of Mexico," said senior vice president Kent Wells, but he cautioned: "We are just starting the test."

He said the oil flow stopped as the last of three valves on a huge capping stack was shut at around 2.25 p.m. (1925 GMT) yesterday, but engineers were keeping a close eye on the operation to see whether any oil began leaking again.

It was the biggest step forward in halting the worst oil spill in US history which has been flowing into the sea since a BP-leased rig sank on April 22, two days after a major explosion on the deepwater drilling platform.

"I'm very excited to see no oil flowing into the Gulf of Mexico," Mr Wells said, adding it was a very "encouraging sign" as the British energy giant works to shut down the oil flow.

Experts have estimated some 35,000 to 60,000 barrels a day have been flowing into the Gulf for some 13 weeks, leaving millions of gallons sloshing around in the waters, threatening vulnerable wildlife and fouling the shores of five Gulf states.

Mr Wells reminded reporters the aim of the pressure tests was to "assess the integrity of the well" as it is not known whether the well-bore which stretches deep below the seabed was damaged in the April 20 explosion.

Over the next 48 hours, engineers will closely monitor the pressures inside the giant 10-metre cap placed on the well earlier this week.

High pressures will show that there is no other leakage underground, whereas low pressures may indicate that the casing of wellbore has cracked and is leaking.