NEW YORK (MarketWatch) -- BP PLC's plans to pressure-test the latest containment system on its blown Gulf of Mexico oil well has received government approval, clearing the way for the next critical step in efforts to tame the leak, a federal official said Wednesday.

National Incident Commander Thad Allen, head of the federal oil-spill response team, said the test will begin later on Wednesday with a gradual buildup of pressure inside the well.

The government had called a 24-hour delay in the test to make sure BP "got it right," according to Allen, drawing on an "overabundance of caution" by first asking a panel of scientists and industry experts whether they believed the well could withstand the enormous pressure of oil and gas rushing from the reservoir deep below the seabed.

The tests will last up to 48 hours, Allen said, during which pressure is expected to reach as much as 9,000 pounds per square inch within the 75-ton containment cap.

A review of seismic data gathered in the area suggests rising pressure levels in the well won't create a crater through which more oil could escape, he added.

President Barack Obama and members of his cabinet were briefed on the plan, Allen said in an afternoon conference call with reporters.

BP BP, -1.18% (BP) will begin to turn off siphoning equipment attached to the mile-deep well and shut the valves on the latest containment cap latched onto the damaged wellhead over the weekend.

Investors, anxious for word on when the next phase of the containment effort could proceed, exerted their own pressure on BP shares. The stock fell nearly 2% Wednesday to close at $36.18.

If BP's well passes the so-called integrity test, the new cap should be able to contain most, if not all, of the 35,000 to 60,000 barrels of oil gushing into the sea since April 20, when the Deepwater Horizon rig drilling the well exploded, killing 11 workers. The rig sank two days later.

If the well casing fails its integrity test, BP will resume siphoning oil from well pipe on the sea floor to collection vessels on the surface.

Regardless, the cap is only a temporary solution. The well won't be fully controlled until BP completes its relief wells, which aim to intersect the blown well thousands of feet below the seabed in what is known in the oil industry as a "bottom kill." That's likely to take several more weeks.

Meanwhile, a spokesman for Sen. Bill Nelson, a Florida Democrat, said Wednesday that BP still has not responded to a June 2 letter inquiring about possible weaknesses in the strength of the well casing.

In one of the many procedures BP initially employed to stop the leak, the company pumped in heavy drilling mud to attempt to clog up the well in May, in an operation called a "top kill."

The letter from Nelson cited a Washington Post report that said new damage was discovered inside the well and that drilling mud leaked out of the well casing.