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.
Perenco UK produces about 40,000 barrels of oil equivalent a day, of which about 14,000 barrels is from Wytch Farm.
Poole Harbour Commissioners (PHC) has activated an oil spill plan and the pipeline had been shut down, with booms placed on either side of the leak, BBC said.
PHC did not immediately respond to a Reuters request for a comment.
Last year, the company's parent company declared force majeure for 150 days following a leak at its Cap Lopez oil terminal in Gabon.
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