An extra 500 troops have been brought in to deal with "unprecedented" flooding in Yorkshire and Lancashire.
Some 400 soldiers are already helping people in areas affected by downpours at Christmas and the PM vowed to help people in "their hour of need".
Emergency teams are evacuating homes in York, where police say 3,500 properties were at risk near the River Foss.
There are scores of flood warnings in England, Wales and Scotland - more than 20 severe, meaning danger to life.
Soldiers are already in action in Cumbria, Lancashire and Yorkshire. The additional personnel are being deployed on Sunday while another 1,500 are on standby in Yorkshire.
After chairing an emergency conference call, David Cameron said the "level of the rivers plus the level of rainfall has created an unprecedented effect, and so some very serious flooding".
"We will do everything we can to help people in this, their hour of need," added Mr Cameron, who is expected to visit some of the flood-affected areas on Monday.
Torrential rain on Friday and Saturday saw rivers burst their banks and waters overwhelm parts of Lancashire and Yorkshire.
Greater Manchester and Leeds are among the areas hit by flooding, with major roads closed and thousands left without power.
BBC Weather forecaster Alex Deakin said areas affected by flooding were enjoying drier conditions but there would be some light rain this evening.
The rain will move away to leave dry and mild conditions at the start of the week, although there is the potential for more stormy weather on Wednesday and a yellow "be aware" warning is already in place for that day for the north of England, Wales and Northern Ireland.
In other developments:
The Scottish Environment Protection Agency has issued a number of flood warnings and alerts, while the Met Office has an amber "be prepared" weather warning in place for heavy rain on Wednesday
Police say West Yorkshire is experiencing its worst flooding in 70 years with more than 2,000 homes in Calderdale and 1,000 in Leeds affected
The River Aire in Leeds, where homes and roads have been flooded, reached "record levels" at 23:00 GMT on Saturday, the Environment Agency said
Flooding from the River Ouse is threatening homes in the village of Cawood near Selby, North Yorkshire
Electricity North West warned power may not be restored to some 6,000 homes until Monday, and urged people in Rochdale to turn off their Christmas lights to help save electricity
Police say the flood operation in Greater Manchester has now turned to "a recovery" phase, as water levels begin to drop
North Yorkshire Police said they had run out of "road closed" signs
The Conwy Valley rail route in north Wales could remain closed for several days after heavy rain caused flooding
Northern Rail is advising passengers not to travel in Cumbria, East Lancashire and West Yorkshire on Sunday
BBC: UK floods: Extra 500 soldiers sent to stricken areas