Successful seismic surveys gave the green light for pressure tests to begin to see if the 75-tonne cap can choke off the leak without threatening the structural integrity of the well.

The device, which contains three giant valves, was lowered late Monday and latched onto the ruptured pipe almost a mile down on the sea floor where only underwater robots can operate.

On the 85th day of the worst US oil spill ever, engineers carried out final checks before gradually closing the valves and shutting down the flow of oil in a process expected to last between six and 48 hours.

High pressure readings would mean there are no other leaks, but low pressures would indicate oil was seeping out of the external casing of the well deep below the seabed.

"Everybody hope and pray that we are seeing high pressures here," said senior BP vice president Kent Wells. "If the tests confirm that we can shut in the well, then the well will obviously be shut in and there will be no leakage into the sea."

Scientists will receive pressure readings every 12 seconds and relay the information to BP and government experts, including Admiral Thad Allen, the former Coast Guard chief leading the US response to the disaster.

President Barack Obama also dispatched Energy Secretary Steven Chu, a Nobel Prize-winning physicist, to oversee the decision-making process.

"We're hopeful," said White House spokesman Robert Gibbs, adding it would be "a big moment" when no more oil was leaking into the Gulf.

The integrity test, however, was not without danger and Wells admitted that pressure caused by closing the valves too quickly on the cap could send oil shooting up from a new leak on the sea floor.

"The worst-case scenario is that it could actually broach back to the sea floor," Wells said.

"The test has been carefully designed to make sure we don't create a bad situation and yet we'll find out the information we need, and find out whether we have the integrity."

Allen said earlier that pressure readings anywhere between 8,000 and 9,000 pounds per square inch would indicate that the casing of the wellbore, which extends 2.5 miles (four kilometers) below the sea floor, is secure.

That would be good news for the Gulf residents who have seen an estimated 35,000 to 60,000 barrels of oil spewing into the sea daily since an April explosion destroyed a BP-leased drilling platform off Louisiana.

Tar balls and ribbons of crude have washed up along all five Gulf states, from Texas to Florida, shutting down key fishing grounds and scaring away tourists.

An estimated 2-4 million barrels of crude have gushed into the sea since the BP-leased Deepwater Horizon sank on April 22, two days after an initial blast which killed 11 workers.

Despite the endgame underway in the Gulf, there was little optimism at a presidential commission hearing into the causes of the disaster as victims struggled to come to terms with the damage.

"Even if BP caps this well tomorrow, they've done so much damage to the Gulf it's a strange consolation plan," Darwin Bond Graham, a sociologist studying how New Orleans has recovered from Hurricane Katrina, said on Monday.

There was anger too at a new moratorium on deepwater drilling in the Gulf with Louisiana Senator Mary Landrieu saying it could cost some 120,000 jobs.

Officials say that even if the new cap cannot seal the well, the capacity of the containment system will be sufficient to capture all the leaking crude.

Whatever happens, BP is continuing to drill two relief wells to intercept and permanently plug the well, with that "kill" operation expected sometime mid-August.

The disaster has cost BP some 3.5 billion dollars (2.78 billion euros) and compensation could mean it ends up forking out 10 times that figure.

BP shares, half their pre-spill value, have risen sharply this week on reports the British energy giant is poised to sell some assets.

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