About 200 barrels of reservoir fluid leak into the Dorset harbour, a site of special scientific interest
The incident, which took place at Wytch Farm oilfield in Dorset, resulted in approximately 200 barrels of "reservoir fluid" being released.
The area is a haven for wildlife and has been designated a site of special scientific interest.
Jim Stewart, chief executive of Poole Harbour Commissioners, said: "A major incident has now been declared and a gold command unit has been established.
"We would advise members of the public that they should not swim in Poole harbour or the surrounding area until further notice.
"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 UK Health Security Agency South West echoed the swimming advice in response to the leak.
Gas company Perenco UK, which operates the Wytch Farm oilfield, confirmed that a "limited oil leak" had occurred from one of its well sites, the pipeline was shut down, and booms were placed on both sides of it to control the spread of the oil.
"Perenco UK confirms that there has been a limited oil leak at one of its well sites in its Wytch Farm operations in Dorset," the firm said.
"Perenco UK's incident management team was activated immediately, the leak was stopped and booms deployed as an additional containment to protect Poole harbour.
"Perenco UK is working closely with the relevant authorities and a clean-up operation is under way."
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."
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."
Philip Broadhead, the leader of Bournemouth, Christchurch and Poole council, said "all relevant partner agencies" are working to address the incident.
"We have today been advised by the operator that there has been a leak from the facility," Broadhead said. "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," he added. "A major incident has been declared which results in all relevant partner agencies working closely together."
"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," Broadhead continued.
"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."
Food water land.