Shares in BP closed down 13% in London on Tuesday after the latest setback in its efforts to contain an oil leak in the Gulf of Mexico.
At one stage, prices were at their lowest level in 15 months.
The BBC reports there are fears the leak will now go on for at least another two months, until relief wells can be drilled.
BP said that the total bill for the clean-up is now $US990 million.
The sell-off in shares was BP's biggest one-day shares fall for 18 years, and 12 billion pounds was wiped from its stock market value.
At its lowest point, the company's share price was nearly 17% down on the day, but it recovered slightly by the close of trading.
Business Monitor International head of oil and gas analysis Holly Pattendon, says the estimated cost of the clean-up is rising.
But she says the real damage is to BP's reputation and brand in the United States.
Worst disaster - Obama
On Tuesday US President Barack Obama called the incident the worst oil spillage disaster in US history.
Eleven rig workers died when the Deepwater Horizon rig exploded on 20 April. It sank two days later.