A species of mussel which is threatening to block water supplies to a US city has been found in the UK.
Quagga mussels have been discovered at Wraysbury reservoir, near Heathrow Airport, the Wildfowl and Wetland Trust (WWT) said.
The molluscs form large colonies on hard surfaces and can block pipes and cause flooding, the trust said.
The Department for Environment, Food and Rural Affairs said it would take action to tackle the threat.
Some scientists say the mussel poses the maximum risk to the UK environment of any non-native species.
The 4cm (1.5in) bivalves originated in Ukraine and spread across Europe as new canals opened, a WWT spokesman said. They were introduced to the US by ships discharging their ballast in the Great Lakes.
