The UK's only wild oyster breeding ground is under threat from contaminated water from a disused mine shaft.
The contaminated mine water could flood into the Fal estuary due to rising water levels at an industrial site near Truro.
It is feared that water from the abandoned mine at Wheal Jane near Baldhu could spill out into the Carnon River and then the Fal Estuary, a designated marine Special Area of Conservation.
The Environment Agency said mine water levels in the shaft were continuing to rise faster than the seven pumps in operation could take the water out.
The Fal estuary, in Cornwall, has one of the few remaining stocks of native oysters in the United Kingdom, and the only one of wild oysters. A law dating to 1868 makes it illegal to gather them with mechanical means or from crafts other than sail boats or row boats.
Mark Pilcher for the Environment Agency, said: "We are doing all we can to monitor the shaft water levels to understand if or when any spill might occur into the Carnon River. The water is currently rising at a rate which is faster than the pumps can manage.
"With the ground saturated after this week's rain, there is a risk that mine groundwater will continue to rise over the next week. The bulk of the mine water will continue to be pumped and treated at the maximum rate but there is a risk that a smaller amount may be discharged and we are working with our partners to assess what the impacts might be."
Cornwall Fire and Rescue Service said it was asked to help pump water from the mine entrance into a dam to alleviate potential flooding and contamination into the Carnon Valley.
The Environment Agency was alerted to the danger by site operator Veolia Ltd.
In normal circumstances, mine water leaving the site is pumped and treated to control pollution.