Hundreds of people were evacuated from their homes today - and part of the M1 motorway closed - after torrential rain led to fears that a dam could burst.
Rotherham council urged residents living near Ulley Dam in South Yorkshire to leave their houses after an expert warned there was a "significant risk" the dam walls could fail after abnormally heavy rain.
Three people have died so far in severe flooding caused by the downpour in the Yorkshire area.
A teenager who was swept to his death in a swollen river in Sheffield was named today as Ryan Joe Parry, 14, who died on his way home from school yesterday.
A 28-year-old man died after becoming trapped in a drain despite a frantic attempt by emergency services to save him.
Mike Barnett was killed after becoming stuck in neck-high water in a storm drain in Hessle, near Hull.
The flooding also claimed the life of a 68-year-old man in Sheffield as he tried to cross a road flooded by water at about 8pm last night.
He was with two other men and was swept away, possibly as he tried to leave his stranded car.
Ryan's body was recovered in the Millhouses Park area of Sheffield after he died in the River Sheaf.
The dam burst warning came overnight. Residents were taken by bus to a temporary evacuation centre set up at Dinnington Comprehensive School in Rotherham.
Council spokeswoman Tracy Holmes said: "We have taken professional advice from an engineer, who said there is a significant risk that the dam could fail.
"Public safety is paramount for us, so we started to evacuate from three specific areas."
Ms Holmes said the decision to evacuate was taken at about 2am.
Residents were advised it was "in their interests" to leave their properties but allowed to remain at home if they were adamant they would not go as long as they stayed upstairs.
Ulley Reservoir covers an area of 35 acres and is about 46ft deep.
By this morning water gushing down the side of the reservoir had caused the part of the earth bank on the dam to collapse.
The nearby M1 was closed in both directions between junctions 32 and 34 because of concerns about the cracks in the dam wall, a Highways Agency spokesman said.
The dam is to the south east of Sheffield, which was overwhelmed by rising flood waters that brought chaos and destruction yesterday.
No homes in Sheffield are expected to be affected if the dam walls burst.
But a power station that serves most of the city lies in the path of the reservoir, a Sheffield City Council spokesman said.
A major rescue operation involving three RAF helicopters was launched as the rising waters trapped workers and motorists in Sheffield last night.
The city was virtually cut off as the floods left office workers stranded in the Brightside Lane area.
About 1,000 people spent the night in temporary rescue centres, either because their houses were flooded or because they were trapped in the city centre and could not get home.
Large parts of the city were left without power as the crisis developed throughout yesterday afternoon and early evening, with added fears of sewage contamination.
The former industrial heartland of the city was submerged under several feet of water as the River Don burst its banks and the drainage system failed with the unprecedented rainfall.
There were reports that several walls and buildings had collapsed because of the water.
The huge Meadowhall shopping centre was closed today as the clean up operation began.
Kelly Darwin, 25, who was stuck on the second floor of her workplace with 15 of her colleagues, said the industrial complex in Brightside Lane was "completely flooded".
Helicopters were airlifting people out of the complex.
"There are cars submerged and bins floating everywhere," she said.
Sheffield's Northern General Hospital said it was coping with the conditions, but a spokesman asked staff who lived nearby to come back to work and cover shifts.
Bob Kerslake, chief executive of Sheffield City Council, said: "We have seen the most intense rain since records began.
"This has been quite unparalleled and extraordinary. People who have lived all their lives in Sheffield would say this is the most severe flooding they've ever seen."
At Dinnington Comprehensive School, more than 350 people were packed into the hall eating breakfast, drinking tea and sleeping.
Centre manager Sam Newton said the centre was full after people were bussed in from all around the area.
She said: "It's all going smoothly at the moment.
"The WRVS has arrived and we're now able to give people some food.
"There are a lot of elderly people here and families but all seems OK at the moment."
Melvyn Stroughair, from Whiston, was eating breakfast with his one-year-old son Harry.
He said: "You just don't think it will happen to you.
"We could not believe it.
"The whole family's here and we're just waiting to be told what's happening."Reuse content