Operations reached a critical phase as engineers raced to take advantage of a stretch of fine weather in the midst of the Atlantic hurricane season and install a new system with the potential to capture all the leaking crude.

Expected to take between four and seven days, the round-the-clock work began at midday on Saturday when the old, less efficient cap was ripped off a fractured pipe a mile down on the sea floor by robotic submarines.

"We are pleased with our progress," BP Vice President Kent Wells told journalists. "We have carefully planned and practised this whole procedure. We've tried to work out as many of the bugs as we can."

A transition spool must first be lowered and bolted onto the leaking pipe before a gigantic funnel -- weighing 150,000 pounds (68 tonnes) and dubbed the "Top Hat 10" -- can be set in position.

The old "Top Hat" collected 25,000 barrels (one million gallons) of crude on average each day, but estimates suggest that could be less than half the total leak.

BP says the new system and the deployment of a third containment ship called the Helix Producer will raise capacity to between 60,000 and 80,000 barrels a day, enough to contain the whole leak.

"We'll capture it all at some point," said Wells.

The White House also expressed optimism.

Senior presidential advisor David Axelrod, appearing on Fox News Sunday and ABC's "This Week" programs, said the administration was "reasonably confident" all of the oil could be contained by the end of July.

"We have every reason to believe that this will work," he said. "We think there is a very, very good chance that this will provide the kind of relief that we need."

The new system is also designed so it can be disconnected and reconnected more easily in the case of a hurricane and has a built-in device that should give the first precise estimate of the overall flow.

No permanent solution is expected until mid-August