People in Oxfordshire are being advised to stay away from a large mass of foam, which is travelling down the River Thames and killing fish in its way.
The foam has been reported to be up to 6ft high in some places
The fire-fighting foam spillage, which can cause irritation to eyes and skin, was first reported at Dudgrove Brook in Fairford, Gloucestershire, on Thursday.
It is travelling towards Oxford and has already killed a large number of fish.
The Environment Agency said it had traced the source of the pollution, but would not say what it was.
Up to 6ft (1.8m) of foam had been reported at Buscot, Radcot and Rushey locks in Oxfordshire.
Emma Bateman, from the Environment Agency, said: "It is not obvious to people while it travels along the main stretches of the water, but it may foam up naturally as it bubbles over the weir, and it can look quite impressive.
"We have received reports that people have suffered from sore throats and itchy eyes if they are near the foam, and if members of the public experience these symptoms they should seek medical advice.
"Fisheries officers are also assessing the impact of the foam on local wildlife and we will continue to do so."
She added that they would not know the exact number of fish killed until the site had been cleared up properly.
And it could take even longer to assess the full environmental impact of the pollution.