A major incident has been declared after an oil leak from an onshore field in Dorset.

The UK Health Security Agency in the South West said members of the public should not swim in Poole Harbour or the surrounding area until further notice after 200 barrels of reservoir fluid, consisting of 85% water and 15% oil, leaked into the water.

The leak has come from the Wytch Farm facility in the Purbecks, which is one of the largest onshore oil fields in Europe.

The facility, taken over by Perenco in 2011, was previously operated by BP.

Perenco said there had been a “limited oil leak” at one of its well sites at Wytch Farm.

The oil and gas company said its “incident management team was activated immediately” and the “leak was stopped”.

However, Philip Broadhead, the leader of Bournemouth, Christchurch and Poole Council, said that although the leak was contained, oil had escaped into the water and surrounding area.

Perenco said it was working closely with the relevant authorities and that a clean-up operation was under way.

Franck Dy, Perenco’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.”

Meanwhile, Mr Broadhead said in his statement: “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.”

Sir Robert Syms, the Conservative MP for Poole, said he had requested the House of Commons Speaker to grant an urgent question on the incident today.

The UK Health Security Agency in the South West said members of the public should not swim in Poole Harbour or the surrounding area, adding: “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.”

Dorset Police said the incident was being “overseen by a number of local agencies”.

Its statement continued: “Please follow the advice not to swim in Poole Harbour or the surrounding area until further notice.”