About 500 barrels of oil have leaked from a pipe off the North Wales coast in the production field known as Liverpool Bay.
The pipe, which runs between two platforms named Conwy and Douglas, was shut off following the leak on Monday and remains closed.
Operator ENI UK said details of what happened about 20 miles (33km) north of Rhyl, Denbighshire, were still being confirmed, but no one was affected.
The firm said there would be updates as details emerged but the Welsh secretary said no oil should reach Wales’ shores.
A leak of about 500 barrels is just under 80,000 litres.
A number of our member companies are on-site and are working with the operator, ENI UK, the Maritime and Coastguard Agency and local authorities who have staff on standby.