Beleaguered BP will find out today if its cap over the oil leak in the Gulf of Mexico is strong enough to last.
The firm completes examinations after temporarily stopping the flow of oil off the coast of America.
The test will see whether the steel casing of the cap is sufficiently strong for the well to be shut off in the long term.
BP already stressed that even if the test proved successful it will not mean the flow of oil and gas has been stopped permanently.
News that the leak was stopped on Thursday helped BP shares soar around 8% higher at one stage yesterday, although wider stock market falls later pared the gains back.
British Prime Minister David Cameron met BP chairman Carl-Henric Svanberg in Downing Street yesterday to discuss the latest developments.
The spill is likely to be high on the agenda in talks with President Barack Obama when Mr Cameron makes his first official visit to Washington next week.
Mr Obama said the company’s breakthrough was “good news”, but warned there was still lots to do.
Mr Obama said efforts would not stop until the leak – the worst in US history - was permanently stopped.
He warned BP would “pay for the damage it has caused” and added caution over early hopes for the progress in capping the leak.
“The American people should take some heart that we’re making progress,” he said. However, he said there was still “a big job to do”.
“We won’t be done until we know we have killed the well and have a permanent structure in place.”
Mr Cameron welcomed the early signs of progress with the cap but said the firm wanted more “certainty” it would not be held liable for problems it had not caused.
“As the BP chairman and I discussed this morning: they want to cap that well and it seems as if they have had, touch wood, some success in the last couple of days,” he told airline workers at an event in Luton.
“They want to clean up the mess, they want to pay compensation to the fishermen and the farmers and the hotel owners and everyone who has been affected.
“But they do need some sort of certainty that the compensation claims won’t go into tertiary and sort of further claims which aren’t really related to the mistakes that BP made.”
BP said last month that it was making a £13bn (€15.4bn) fund available for compensation.
The stock dipped below 300p at one stage last month – its lowest point since August 1996 – but recovered to more than 400p amid signs that it is closer to tackling the crisis, which has so far cost more than $3.5bn (€2.7bn) in spillage and clean-up.
The disaster began when the Deepwater Horizon rig exploded on April 20, killing 11 workers.
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 (€6.9bn) 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 (€127m) in claims to 52,000 businesses so far, although the intense political pressure on the oil giant eased after it 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.”
Relief wells being drilled miles beneath the seabed will be the only means of permanently sealing and isolating the damaged well. The first of the wells is expected to be finished by the end of the month.
It is thought more than four million barrels of oil have so far flowed into the Gulf.