The former Coast Guard chief leading the US response to the disaster made the decision to put the test off until at least Wednesday after meeting Energy Secretary Steven Chu, a Nobel Prize-winning physicist, and other top experts.

BP is poised to test whether a huge 75-tonne cap can seal the leak without threatening the structural integrity of the well.

But US officials fear that if the pressure caused by shutting off the flow of oil increases too quickly, it could send oil shooting up from a new leak on the sea floor.

"We continue to prepare and review protocols for the well integrity test -- including the seismic mapping run that was made around the well site," Admiral Thad Allen said in a statement late Tuesday.

"As a result of these discussions, we decided that the process may benefit from additional analysis that will be performed tonight and tomorrow," Allen said.

BP's capping 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.

Once given the go-ahead, BP engineers will gradually close the valves to lock in the oil, in a process expected to last between six and 48 hours.

BP put out a statement shortly after Allen, confirming the test had been put off until more checks could be done.

"This analysis will be conducted tonight and into tomorrow (Tuesday night and Wednesday). Consequently, the well integrity test did not start today," the statement said.

Once the test gets under way, fingers will be crossed in the hope of high pressure readings which would mean there are no other leaks and the valves on the cap can remain closed to effectively seal the well.

Low pressure would indicate oil is seeping out of the external casing of the well, meaning the valves would have to be reopened to reduce the risk of a new gusher on the seabed. Containment operations would then have to resume.

"The worst-case scenario is that it could actually broach back to the sea floor," explained 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 Allen and Chu, who was dispatched by US President Barack Obama to oversee decision-making for this pivotal moment in the 85-day disaster.

Allen has said 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 explosion destroyed a BP-leased drilling platform off Louisiana in April.

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 key to local economic health.

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

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.

Despite the endgame under way in the Gulf, victims voiced their dismay at a presidential commission hearing into the disaster this week.

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

The Gulf 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 however on reports the British energy giant is poised to sell some assets.

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