BP has put its stake in western Europe's biggest onshore oil field - in Poole Harbour, Dorset - up for sale.
The BP-operated Wytch Farm site was discovered in the Wessex Basin in 1973 by the British Gas Corporation.
BP is selling its 68% stake at Wytch Farm and its stakes in the nearby Kimmeridge and Wareham oil fields.
BP said there was already a number of interested parties looking to take over the operations.
The firm said it expected staff currently working at the sites to transfer their employment to the new buyer, once the sale was completed at about the end of the year.
It added the divestments would allow BP to "focus resources and investment on its diverse central North Sea, northern North Sea, West of Shetland and Norway assets and on successful delivery of its new major projects".
Trevor Garlick, BP's regional president, said: "The assets we intend to divest are of high value but find it difficult to compete for capital and resource within our North Sea portfolio.
"We believe they will attract earlier investment and be of greater value to a new buyer."
He said ensuring the continued safe operation of the sites would be the firm's priority.