Thousands of people are being evacuated from a town because part of a reservoir wall has collapsed in floods.
Engineers are pumping water from the 300-million-gallon Toddbrook Reservoir amid fears it could burst and swamp Whaley Bridge in Derbyshire.
Police have told the town's 6,500 residents to gather at a school, taking pets and medication with them.
A severe flood warning, which means a threat to life, has been issued for the River Goyt below the reservoir.
Julie Sharman, chief operating officer of the Canal and River Trust, said: "The spillway for the dam has broken away.
"We are trying to lower the level of the water in the dam in order to reduce the risk of further erosion. There's a risk the dam could break."
'Very real danger'
Det Ch Supt Rachel Swann, of Derbyshire Police, said a multi-agency taskforce had drawn up a plan to address damage to the wall.
Engineers would continue to pump water from the reservoir, she said, and a Chinook helicopter would be used to drop 400 tonnes of aggregate on "surrounding watercourses".
This would "divert water from entering the reservoir," she said, so the water could be reduced to a safe level.
Once this was done, Det Ch Supt Swann said, work on the wall of the dam could begin.
"With all that said, at this time the future of the dam wall remains in the balance and I would remind people of the very real danger posed to them should the wall collapse," she added.
Ms Sharman, of the trust, said officials could not say how long the evacuation could last but they could be working on it "for days".
Ten pumps from fire services across the country are currently pumping out 7,000 litres of water a minute in a bid to bring down the water level.
Assistant Chief Constable Kem Mehmet said police had plans for every scenario including the dam, which holds back 1.3 million tonnes of water, collapsing.
Police told residents to gather at Chapel High School in the neighbouring town of Chapel-en-le-Frith and to take pets and medication with them as they "are not sure how long [it] will take".
They added it was "an unprecedented, fast-moving, emergency situation", and confirmed a small number of houses in the neighbouring villages of Furness Vale and New Mills have also now been evacuated.
They said evacuated residents should try to arrange their own accommodation as space was limited, but some local pubs and community halls had offered to take in evacuees.
The Environment Agency has said about 85% of people in Whaley Bridge have left their homes.
Area director Lee Rawlinson said community spirit has been "fantastic".
"A lot of people have gone up to Chapel or to other relatives. Whaley Bridge can be really proud," he said.
Meanwhile, trains between Hazel Grove and Buxton have been blocked because of the flooding, affecting Northern train services between Manchester Piccadilly and Buxton.
'Bring the dog'
Jennifer, owner of the Goyt Inn, has been evacuated and is currently in the school.
She said: "I rang my partner and said, 'Bring the dog. We have to get out'.
"We've been told if we need accommodation, something will be organised, but at the moment, [the council] are waiting on structural reports from engineers.
"The dam is a mess. It really looks very unsafe and there's a lot of water in that reservoir."
Nigel Carson, who lives near the dam, said he felt some people in the area were not taking the problem "as seriously as they should".
"The whole of the village is vulnerable to this," he said.
"They're estimating that without it raining it will take two to three days to get the water level down two metres [but] it's raining heavily again now, and they need to get it down an awful lot more than two metres to make this safe."
Duncan Fife lives on top of the reservoir in the old Reservoir Keeper's house, said: "We have been telling them for years that the spillway has been clogging up with plants trees and weeds. Just now they are starting to clear it. Why don't they maintain it?"
But he added the water has now dropped "a couple of feet" since last night.
'People abandoning vehicles'
Symon Trelfa, whose home is near the dam, said he is "worried I may have no home to return to".
He said: "There were rumours going around about the state of the dam yesterday so I decided to move into my sister's house in Chapel-en-le-Frith.
"There's gridlock on the local roads at the moment - people are abandoning vehicles."
Former Conservative MP Edwina Currie also lives in the town.
She said: "Evacuation is absolutely necessary. I don't think they've got any choice. People are being advised not to take pictures. We are really very, very worried."
Rebecca Morelle - Science correspondent
The dam is quite old - it was built in 1831 and is an earth-filled construction.
The problem is the damage is high up. The key is to get the water levels down as fast as you can.
You can divert it out through channels and there is a plug deep down.
The key will be to see if it holds overnight. If it does, there's a good chance they can repair the dam but it will take months.
Councillor Martin Thomas, chair of Whaley Bridge town council, told the BBC the clay under the slabs of the spillway had been undermined when they were lost.
He said: "The situation is an unfortunate combination of circumstances.
"We've had a lot of wet weather recently, the ground is saturated, the reservoir was already full.
"Something we previously looked at as a benign leisure facility suddenly becomes a real mortal danger."
Ruth George, Labour MP for the area, said the town could be evacuated for "several days".
"It's hard to see how they can get the water in the reservoir down quicker than that," she said.
"The dam had some pretty significant structural repairs a few years ago.
"We were told after that it was safe but we have had an unprecedented amount of water coming down in the past few days.
"Nobody is looking at laying the blame. What we want to do is make sure the people in the town are safe."
The Canal and River Trust said it is working with "other agencies" and had engineers trying to get water out while sandbags are being put on the edge to shore it up and protect the rest of the concrete.
When asked if the dam might "actually break" a spokesman responded "there is a risk of that".