NEW ORLEANS — BP plugged its runaway well in the Gulf of Mexico with cement Thursday, one of the final steps in permanently sealing the gusher at the center of the worst U.S. environmental disaster on record.

“This is not the end, but it will virtually assure us that there will be no chance of oil leaking into the environment,” Coast Guard Admiral Thad Allen told reporters.

In a long-awaited breakthrough, BP brought the well under control Wednesday after pumping heavy drilling fluid into the busted Macondo well for eight hours, forcing the oil back down into the reservoir miles beneath the seabed.

The British energy giant then began pumping cement at 10:15 a.m. EDT, and the “static kill” operation was completed in five hours.

“Monitoring of the well is underway in order to confirm the effectiveness of the procedure,” BP said in a statement.

The final phase will involve pumping mud and cement through a relief well into the outer ring, or annulus, of the well, and possibly into the well casing.

“Depending upon weather conditions, mid-August is the current estimate of the most likely date by which the first relief well will intercept the Macondo well annulus,” BP said.

It took 106 days to shut the well down in the wake of a devastating explosion on April 20 that killed 11 workers and sank the BP-leased Deepwater Horizon rig, unleashing a torrent of oil into the Gulf. At 4.9 million barrels — or enough oil to fill 311 Olympic-sized swimming pools — the disaster is the biggest maritime spill on record.

“It’s impossible to know how this thing is ultimately going to play out,” said Matt O’Brian, owner of a shrimp and crab processing dock in the coastal town of Venice, La.

O’Brian welcomed the news that the well was finally under control, but said it “can’t overcome the atmosphere of uncertainty lingering out at sea.”

He’s worried about the oil’s impact on crab and shrimp populations and wonders if there will ever be a market for Louisiana seafood.

Todd Goodman, who works for the local government and runs a trailer park as a sideline, agreed.

“There is enormous pressure on BP to claim that everything is fine now. But what scares me and a lot of other folks around here is the notion that everybody — BP, the Coast Guard, law enforcement, cleanup crews — will suddenly pull up stakes and leave,” he said. “Then, two months later — boom! — more oil washes up on us.”