Anglo-French oil company Perenco's UK unit said on Sunday that a limited oil leak occurred at one of its well sites in Wytch Farm in Dorset, southern England.
Perenco UK said the spill was being contained and an investigation will be launched.
"Any spill is an extremely serious matter and a full investigation will be launched to ascertain what happened in Poole Harbour," Perenco UK's Wytch Farm General Manager Franck Dy said in a statement.
BBC reported that a major incident was declared after about 200 barrels of reservoir fluid leaked into the water at Poole Harbour.
Jim Stewart, the chief executive of Poole Harbour Commissioners (PHC), an independent body that regulates activities of the harbour, told BBC Radio that Perenco now estimated the quantity at less than 200 barrels, and said the liquid was 80% saline solution and 20% crude.
Stewart said the spill had been designated one which required a regional rather than national response.
Perenco UK produces about 40,000 barrels of oil equivalent a day, of which about 14,000 barrels is from Wytch Farm.
PHC said it activated an emergency oil spill plan and the pipeline had been halted, with booms placed on either side of the leak.
PHC also warned that people should avoid using beaches within Poole Harbour until further details are available.
Last year, Perenco UK's parent firm declared force majeure for 150 days following a leak at its Cap Lopez oil terminal in Gabon.
(Reporting by Maria Ponnezhath in Bengaluru and Shadia Nasralla; editing by Cynthia Osterman and Jason Neely)
