Hundreds of people have fled their homes in three villages in South Yorkshire amid fears a dam is about to burst after torrential rain.
Engineers are trying to reinforce the wall holding back the Ulley reservoir near Rotherham after cracks appeared.
A 68-year-old man and a teenage boy died in separate incidents in Sheffield and a 28-year-old man was killed after becoming stuck in a drain in Hull.
Elsewhere weather conditions are easing after severe flooding across England.
But Sheffield remained virtually cut off from the rest of the country as the transport infrastructure all but ground to a halt. Bus and rail services are severely disrupted and many major roads are closed.
City council chief executive Bob Kerslake said: "This flooding has caused severe disruption to the highways network in Sheffield, including road closures and a major strain on the public transport network.
"This has meant that many people have found it very difficult to travel into and out of the city."
About 100 homes in the villages of Catcliffe, Whiston and Canklow were cleared by police officers overnight after engineers signalled a warning over the dam wall of the 35-acre Ulley reservoir, in a country park south of Rotherham.
Fire crews are pumping water out of the reservoir in a bid to ease the pressure on the dam.
The M1 motorway has been closed between junctions 32 and 36 because it would be flooded if the dam burst.
South Yorkshire's Chief Constable Meredydd Hughes said: "It's not just the fact that the dam might break and the wall of water will rush to the motorway, it's also the fact that that water may take out a couple of pylons that are carrying 270,000 volts of power."
Asked about the chances of the dam bursting, Ch Supt Matthew Jukes, of South Yorkshire Police, said: "It's pretty difficult to tell at the moment to be absolutely frank.
"The situation is stable and there are some fine judgements being made, and obviously in the context of making those fine judgements we're taking that cautious approach to keeping people safe.
"We have taken people away from their homes, so hopefully the risk to life will be very limited, but clearly there's a substantial risk of damage to property and that's the one we're trying to mitigate."
Prime Minister Tony Blair sent his condolences to the families of people who died in the floods.
"This has been an extraordinary and very serious event for us," said Mr Blair.
"We are in very close touch, obviously, with the authorities there. The Gold Command has been set up. The emergency services are doing a fantastic job."
About 900 people are in emergency shelters in Sheffield after being left stranded by the flooding.
At a news conference on Tuesday, police and city council representatives paid tribute to the "resilience" of the emergency services and the people of Sheffield.
They said there had been "significant damage" to the city. No figure has yet been calculated to assess the cost of the damage, but it would run into "millions".
The scenes were described as "unprecedented" and, in the view of the Environment Agency, "off the scale", it was said.
Fears over the dam followed chaotic scenes across the Sheffield area caused by torrential rain on Monday which turned roads into churning rivers of flood water.
A teenager who was swept to his death in a swollen river has been named as Ryan Joe Parry, 14, who died on his way home from school.
His body was recovered in the Millhouses Park area of the city after he died in the River Sheaf.
Ch Supt Jon House, of South Yorkshire Police, said: "Sadly, it appears that Ryan was on his way home from school with friends, because it was such an exceptional event yesterday, they went over to look at the river.
"They, I think, went a little too close... and unfortunately and very sadly, Ryan was taken into the river."
About 100 people took shelter at Dinnington Comprehensive School
A 68-year-old man also died as he was washed away as he got out of his car in the Wicker area.
Shocked Sheffield residents described their experiences during the height of the floods.
Jeff Wilkes was rescued from the engineering firm Sheffield Forgemasters after being trapped by rising flood waters.
"We looked out on to Brightside Lane and there were cars floating down and we were told to get into the office block which is a three-storey building.
"The water was getting higher and higher and we couldn't get out. We expected we'd have to stay there all night but the next thing a fireman came in a boat and got us out."
Lisa Thompson and her four-month-old baby were rescued by a digger from a nearby building site after becoming trapped in a multi-storey car park at Meadowhall shopping centre.
"My husband and mother-in-law and family were stuck in the multi-storey.
"I was rescued by a JCB, believe it or not. Because my baby's four months old I was the first person to be rescued."
About 13,000 people in Sheffield have been left without power because of the floods.
Power company YEDL said its engineers may need to wait until the water subsides before they can get the remaining homes reconnected.
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