Poole Harbour Commissioners (PHC), who regulate activities on the harbour, said a leak occurred at a pipeline operated by gas company Perenco, under Owers Bay on Sunday.
Perenco, the UK’s largest onshore oil field, said a “small” amount of reservoir fluid (consisting of 85% water and 15% oil) escaped from its pipeline and that, as of late Sunday night, some of it had already been recovered.
While “early indicators are that the surface slick is already dispersing”, PHC said.
The commissioners added that Perenco had reported “no risk” of further leakage after the pipeline was shut down and booms were placed on either side of the leak to minimise pollution.
A further assessment will be made early on Monday morning regarding the clean up operation, PHC said.
Anyone who came into contact with the spill was being urged to “wash immediately with soap and water”.
Approximately 200 barrels of reservoir fluid was released into Poole Harbour after the leak occurred at Wytch Farm oil field in Dorset on Sunday.
According to Poole Tourism, Poole is Europe’s largest natural harbour and a site of nature conservation, with many international protections in place.
The harbour is also a Ramsar site which recognises wetlands of international importance, particularly for wildfoul, a Site of Special Scientific Interest (SSI) and a Special Protection Area (SPA).
Franck Dy, Perenco UK’s Wytch Farm general manager, said: “Any spill is an extremely serious matter and a full investigation will be launched to ascertain what happened in Poole Harbour.
“It is important to stress that the situation is under control, with the discharge of fluids having been stopped and the spill is being contained.”
The firm confirmed a clean-up operation is under way.
Philip Broadhead, the leader of Bournemouth, Christchurch and Poole Council, said in a statement: “The Wytch Farm oil field in the Purbecks has been operating since 1979 and is one of the largest onshore oil fields in Europe.
“We have today been advised by the operators that there has been a leak from the facility. Whilst this has been contained, we are told that oil has escaped into the water and surrounding area.
“We are liaising closely as part of long-established mechanisms with a variety of parties as this situation unfolds.”
An Environment Agency spokesperson said: “We have received reports of a large amount of oil in Poole Harbour and we are supporting the port authority and other partners in responding to this incident.”