The ruptured well which pumped almost five million barrels of oil into the Gulf of Mexico has been secured and no longer constitutes a threat, it was confirmed tonight.
A new valve known as a blow-out preventer was placed over the well yesterday after crews replaced a damaged device.
This will now be examined by investigators looking into the causes of the disaster.
Incident Commander Admiral Thad Allen said: “Under the direction of the federal science team and US government engineers, BP used the Development Driller II to successfully install a fully functioning and tested Blow Out Preventer (BOP) on the cemented Macondo 252 well.”
He said there was no observable release of hydrocarbons during the operation.
“This is an important milestone as we move toward completing the relief well and permanently killing the Macondo 252 well. I will continue to provide updates as necessary,” he added.
The ruptured well has been shut since July 15 after the Deepwater Horizon rig exploded killing 11 workers and causing the worst environmental disaster ever to hit the region.
Since the accident on April 20, BP has come under prolonged attack in the US for perceived safety failings and its attitude towards the investigation.
Besides causing catastrophic pollution, the oil spill also led to the departure of BP chief executive Tony Hayward.
The oil giant has estimated the cost of tackling the disaster to eight billion US dollars (£5.2 billion) so far.