Oil giant BP says it has made a "giant" new oil discovery in its fields in the Gulf of Mexico.
BP is currently the largest producer of oil and gas in that area, with net production equivalent to more than 400,000 barrels of oil a day.
The company said it had drilled the well, dubbed Tiber, to a total depth of about 35,055ft (10,685m), making it one of the deepest wells drilled to date.
BP shares rose 4.3% to £5.41, making it the biggest gainer in the FTSE 100.
Tiber represents BP's second material discovery in the Lower Tertiary area of the Gulf of Mexico, after Kaskida.
BP said the discovery, amounting to more than three billion barrels, would "support the continuing growth of our deepwater Gulf of Mexico business into the second half of the next decade".
The oil firm will now undertake surveys to determine the oil field's size and commercial potential.
The industry-wide definition for a "giant" discovery is at least 250 million barrels of oil "in place", or in other words, the likely total amount, BP spokeswoman Sheila Williams said.
But usually, only as much as 30% is extracted from the ground in practice, she said.
BP first started drilling in the Tiber well in March.
The oil firm controls 62% of the Tiber, along with 20% held by the Brazilian state-controlled company Petrobras and US firm ConocoPhillips with 18%.
It has nine projects in various stages of development in the Gulf of Mexico.