The future of a dam at risk of bursting in Derbyshire "remains in the balance" after thousands of people were evacuated amid fears lives could be in danger.
An RAF Chinook has been dropping one-tonne sandbags to shore up the damaged reservoir wall in Whaley Bridge.
Heavy rain led to what police called an "unprecedented, fast-moving, emergency situation".
Boris Johnson said he had asked Environment Secretary Theresa Villiers to chair a meeting of the Cobra emergency committee to discuss what was happening.
In a tweet, the prime minister said his thoughts were with all those affected and "first responders, engineers and RAF crews are working around the clock to fix the dam".
Toddbrook Reservoir, which contains 1.3 million tonnes of water, sits above hundreds of homes and businesses in the town.
Water flowing into the dam was "reduced considerably" overnight but engineers remain very concerned.
Julie Sharman, head of the Canal and River Trust which runs the reservoir, said water levels had reduced by around 8in (20cm).
More than 6,000 people live in the town, 16 miles southeast of Manchester.
The Chinook is helping move more than 400 tonnes of aggregate into place.
Deputy Chief Constable Rachel Swann said: "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."
Water pumps that can flush out up to 7,000 litres of water per minute are also being used to relieve pressure on the wall.
Derbyshire Fire Service said 150 firefighters were at the scene.
Ms Swann said the decision to evacuate homes was "very difficult" and she did not know when residents would be able to return.
Sky News correspondent Tom Parmenter said on Thursday night that police officers were "running at us screaming and ordering people to retreat to higher ground" - but it appeared to have been a false alarm.
Resident Carolyn Whittle said another section of the spillway had collapsed on Thursday.
"I've never seen water flood over the dam like that, ever, nor thought that we could possibly be at risk in this way," said Ms Whittle.
Former MP Edwina Currie, who lives in a cottage above the reservoir, told Sky News the area had seen two weeks' rain in 24 hours.
She said strong winds on Wednesday had caused the dam to overflow, which she described as a safety feature and nothing to be concerned about.
However, the force of it had carried away some of the dam's concrete facing.
"The community here is excellent... absolutely buzzing with people offering help," she said.
"I've no doubt everyone will get well looked after."
Resident Anna Aspinall said she and others had been called to help place sandbags but had been sent away after structural engineers said "the wall is at high risk of failing".
She added: "We are praying (the dam wall) holds whilst the Canal and River Trust try to drain the water from the reservoir."
Paul Nash said: "The River Goyt is actually behind us, normally it's 20ft down from our back garden but last night it raised up to nearly 3ft from coming over.
"If the whole dam goes, it's going to cause absolute chaos. Probably the village will go, because it goes straight through. The River Goyt goes straight through the village centre."
In other weather-related news:
- Police have closed railway lines in the Whaley Bridge area amid concerns the dam will burst and cause flooding, East Midlands Trains said.
- Flooding has damaged homes and partially destroyed a bridge in Poynton, Cheshire, forcing some to head to a rest centre at the local civic hall. Firefighters responded to 20 incidents, including people stranded by flooding.
- In nearby Wilmslow, police are "dealing with flooding" and had evacuated affected residents, with Oakenclough Children's Centre open as a rest centre.
- The Environment Agency said its teams were in Cheshire, Derbyshire and Greater Manchester, where intense rainfall had caused flooding and disruption.
- There is currently one severe flood warning (relating to Whaley Bridge) in force, six flood warnings meaning flooding is expected and immediate action is required (two for Poynton Brook, three for the River Bollin and one for Timperley Brook at Altrincham) and 14 flood alerts, meaning flooding is possible.
- The Met Office says sporadic rain is expected in northwest Scotland and southwest England on Friday but will not be as frequent or as significant as in recent days.