Hundreds of fish have been found dead at a seaside bay classified as an internationally important wetland.
Raw sewage is thought to have killed 400 fish in the bay
Raw sewage is thought to have entered the water, the Environment Agency said.
Officers were called to Wallers Haven at Normans Bay, near Eastbourne, East Sussex, after a call from the public alerting them to dead fish.
The full extent of the damage has not been established but it is estimated that at least 400 fish have died. An investigation is under way.
Spokeswoman Liz Wood said the agency took such incidents "extremely seriously".
She said the agency was "launching a full and thorough investigation to establish exactly how sewage entered Wallers Haven and if this is what has killed these fish".
And she added: "Unfortunately it looks like hundreds of fish have died at this internationally recognised site.
"Sewage entering a river or stream can have a devastating effect on the wildlife living there."
She explained that nutrients in the sewage could break down and use up the oxygen in the water, making it hard for fish to breathe.
"This is an unfortunate reminder to anyone who handles polluting matter that they take care to ensure that there is no risk to the environment," she said.
Wallers Haven is designated as a Site of Special Scientific Interest.
It is also a Ramsar site of international conservation importance - under a convention adopted in Ramsar, Iran, in 1971, Ramsar sites specifically protect important wetlands.