Sunday's spill of about 200 barrels of reservoir fluid came from a pipeline operated by gas firm Perenco.

The company said the "small" leak was quickly shut off and that it had already recovered some of it by Sunday night.

An assessment is due today but Poole Harbour Commissioners (PHC) said they believe the "surface slick" is already dispersing.

The UK Health Security Agency urged the public not to swim in the harbour or surrounding area, while PHC said the beaches within the harbour should also be avoided for now.

Anyone who comes into contact with the fluid - made up of 85% water and 15% oil - is being advised to wash immediately with soap and water.

Poole is Europe's largest natural harbour and a nature conservation site, according to the town's tourism agency, and some environmental groups have claimed the incident will have a "devastating impact".

The leak came from the Wytch Farm oil field in Dorset but the pipeline affected was quickly shut down and booms put on either side to try to contain it.

Franck Dy, Perenco's site 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."

Dorset Police said the incident was being "overseen by a number of local agencies" and reiterated the warning not to swim "until further notice".

Council leader Philip Broadhead said officials were liaising "with a variety of parties" but stressed the leak had been contained.

Local MPs Sir Robert Syms and Richard Drax are set to ask the government an urgent question about the spill in the Commons later.

Alice Harrison, fossil fuels leader at the Global Witness campaign group, said: "Whilst the extent of the damage is still unknown, there is no doubt that a huge spill like this will have a devastating impact on the local population and environment in Dorset."