BP was today waiting to see whether the capped-off well which has spewed oil into the Gulf of Mexico since April will hold.
The company confirmed last night it temporarily stopped the flow of oil which so far cost $3.5bn (€2.7bn) in spillage and clean-up.
The well was sealed with a test cap that should stop crude oil spilling into the ocean for 48 hours.
But BP stressed that even if the test is successful it will not mean the flow of oil and gas has been stopped permanently.
BP executive Kent Wells said the oil stopped flowing at 7.25pm Irish time.
He added: “It is very good to see no oil go into the Gulf of Mexico.”
President Barack Obama called it a positive sign, but cautioned: “We’re still in the testing phase.”
The disaster began when the Deepwater Horizon rig exploded on April 20, killing 11 workers.
Shares in the oil giant were hammered by the disaster but have shown signs of recovery in recent days amid speculation of a takeover by US rival ExxonMobil and hopes of progress on tackling the crisis.
BP is vital to the UK not only as an employer but also a major taxpayer and contributor to pensions; of every £7 paid into pensions from dividends £1 comes directly from BP and last year it paid £5.8bn (€7bn) in taxes.
This year BP is not paying a dividend for the first time since the Second World War, although it insists it is financially strong enough to tackle the spill.
The group paid out about $165m (€774m) in claims to 52,000 businesses so far, although the intense political pressure on the oil giant eased since the group set aside a $20bn (€15.5bn) compensation fund to meet the costs of the disaster in June.
A BP spokeswoman said: “Information gathered during the test will be reviewed with the relevant government agencies, including the federal science team, to determine next steps.
“The sealing cap system never before has been deployed at these depths or under these conditions, and its efficiency and ability to contain the oil and gas cannot be assured.”