British police ordered the evacuation of a town of 6,500 people in north-west England on Thursday over fears that a rain-damaged dam could collapse.
The Derbyshire Police force said residents of Whaley Bridge should leave their homes immediately because of the "significant threat to life".
It said a dam wall at the 19th-century Toddbrook Reservoir above the town had been damaged by days of heavy rain.
Police officers went door to door in Whaley Bridge telling people to head for a school a few kilometres away.
Residents of the town, 280km north-west of London, were advised to stay with friends and family elsewhere if possible, and to take pets and several days' worth of any essential medications.
"This is not a decision that has been taken lightly and we appreciate that there is significant impact on this community," police tweeted.
"However, this is an unprecedented, fast-moving emergency situation."
A heatwave last week has been followed by heavy rains in many parts of the UK, causing flash floods that have inundated homes, roads and train lines across northern England.
Whaley Bridge resident Anna Aspinall said engineers had told residents that the dam wall was at a high risk of failing.
"We are praying it holds while the Canal and River Trust try to drain the water from the reservoir," Ms Aspinall said.
Paul Nash, who was moved to safety with his partner and their cat, said police had given them no indication of how long they needed to be away from home.
"If the whole dam goes it's going to cause absolute chaos," Mr Nash said. "Probably the village will go because it (the River Goyt) goes straight through."
