BP has said the cement sealing the wrecked oil well in the Gulf of Mexico had hardened as crews prepared for the final phase of drilling a relief well to end to one of America's worst oil spills.
The oil giant said pressure tests on the cement plug poured down the throat of the blown-out well showed the seal was solidly in place.
That means BP engineers can begin drilling the final 100 feet of a relief well meant to permanently seal the blow-out.
Crews will carefully drill about 30 feet at a time and BP says it will probably be next weekend before the two wells meet.
Engineers will use the relief well for a "bottom kill", pumping more mud and cement into the well in what is expected to completely seal it for good.
The Deepwater Horizon rig exploded on April 20, killing 11 workers.
Scientists released a report saying that only about one-quarter of the nearly 207 million gallons of oil that leaked into the Gulf of Mexico was still oozing around the sea. Even the amount left, though, would still rank among America's worst spills.
BP left open the possibility that it could some day drill a new path into the same undersea reservoir of oil, still believed to hold nearly £2.5 billion worth of crude.
But the US Interior Department warned that the blown-out well, a relief well and a back-up being drilled to help stop the leak would not themselves be used to pump new oil out of the reservoir.
"The well is almost dead. Under no circumstances are we going to allow them to reopen the well to extract oil and gas," spokeswoman Kendra Barkoff said.