A clean up operation has begun after 200 barrels of reservoir fluid leaked into the water close to Sandbanks.
A major incident was declared on Sunday after around 200 barrels of reservoir fluid including oil leaked into the water at Poole Harbour, Dorset.
Poole Harbour Commissioners (PHC), an independent body that regulate the activities of Poole Harbour, said that the leak occurred at a pipeline managed by Perenco, under Owers Bay.
Perenco said its incident management team had been activated immediately to stop the leak while a clean-up operation was underway.
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."
Sightings of oil in the harbour following the spill are "down by 60-70%," according to PHC, as a clean-up plan to protect sensitive nature reserves is being implemented.
The public is still being advised to avoid going into the water or using the beaches despite the oil appearing to begin to disperse.
Perenco said that a "full investigation" would be carried out into how the leak occurred.
Poole is the largest natural harbour in Europe. It is also a Site of Special Scientific Interest (SSI) which is home to a variety of wading birds.
PHC said: "It is estimated that approximately 200 barrels of reservoir fluid has been released into the water column in Poole Harbour.
"Anyone who has come into contact with the spill should wash immediately with soap and water. If eyes have come into contact with the spill, they should be rinsed with water."
The commissioner said it had also put its oil spill plan into place while the pipeline had been shut down, with booms placed on either side of the leak.
PHC added that companies specialising in oil spill response had also been mobilised.
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."
Mr Broadhead's statement added: "As leader of the council, I will, along with the senior council team, be working to see what implications this leak may have and what action we may need to take.
"It is of course extremely disappointing to hear of this event, and while there will be a time later for anger and investigation, our focus now must be on ensuring we can mitigate any impacts of this situation."
