BP has allayed to allay last-minute US government fears of making the disaster worse by started trying to slowly choke off the flow of oil into the Gulf of Mexico, in the hope of finally stopping the leak.
Retired Coast Guard Admiral Thad Allen, the Obama administration's point man on the disaster, said the government gave the go-ahead after carefully reviewing the risks of the procedure.
The plan is a test of whether a new temporary well cap can withstand the pressure and ultimately contain the oil.
"What we didn't want to do is compound that problem by making an irreversible mistake," he said at the end of a 24-hour roller-coaster of hopes raised, hopes dashed and hopes raised again along the Gulf Coast.
The cap - a 75-ton metal stack of pipes and valves - was lowered onto the well on Monday in hopes of either bottling up the oil inside the well machinery, or capturing it and funnelling it to the surface.
But before BP could test the equipment, the government intervened because of second thoughts about whether the buildup of pressure from the gushing oil could rupture the walls of the well and make the leak worse.
"We sat long and hard about delaying the tests," Admiral Allen said. He said that the pause was necessary in the interest of the public, the environment and safety, and that officials were convinced the test could go forward.
If the cap works, it will enable BP to stop the oil from gushing into the sea, either by holding all the oil inside the well machinery like a stopper or, if the pressure is too great, channelling some though lines to as many as four collection ships.
The test began with BP shutting off pipes that were funnelling some of the oil to ships on the surface so the full force of the gusher went up into the cap.
Then deep-sea robots began slowly closing, one at a time, three openings in the cap that let oil pass through. Ultimately, is is hoped that the flow of crude will be blocked entirely.
All along, engineers will be watching pressure readings to learn whether the well is intact. The first two valves shut off like a light switch, while the third works more like a dimmer and takes longer to close off.
Yesterday evening, the company said it isolated a leak on the line attached to the dimmer switch, and was repairing it before moving forward. It was not clear how that would affect the timing of the operation.
Admiral Allen said BP will monitor the results every six hours and end the test after 48 hours to evaluate the findings.