BP has permanently plugged the ruptured Macondo well, which has spewed more than four million barrels of oil into the sea since April and caused substantial damage to the environment.

BP confirmed that the well kill operation on the Macondo well in the Gulf of Mexico had been completed, with both the casing and annulus of the well sealed by cement.

National incident commander Admiral Thad Allen said the Macondo 252 well was effectively dead and posed no continuing threat to the Gulf of Mexico.

The relief well intercepted the annulus of the MC252 well, followed by the pumping of cement and a pressure test, which showed that the well was permanently clogged.

BP said that it would now start the process of abandoning the well, dismantling and recovering containment equipment and decontaminating vessels that were in position at the wellsite.

The oil spill prompted a ban on deepwater drilling and has cost the offshore industry millions of dollars.