Heavy rain has caused more flooding in northern England, with homes evacuated in Lancashire and Yorkshire, and rivers overflowing in Manchester and Leeds.
Vulnerable people are being evacuated from 2,200 homes in York, where people in 3,500 properties near the River Foss have been advised to be ready to leave.
Six severe flood warnings - indicating a danger to life - are in place.
Met Office warnings of further rainfall are in place for northern England, Scotland and Wales.
In total, hundreds of flood alerts and warnings have been issued - more than 30 of them severe.
Prime Minister David Cameron tweeted: "My thoughts are with people whose homes have been flooded.
"I'll chair a COBRA [emergency committee] call [on Sunday] to ensure everything is being done to help."
The River Irwell and River Roch have both overflowed, causing flooding in Salford, Manchester city centre and Rochdale.
Roads in central Leeds, where a severe warning is in place on the River Aire, are also under water.
Red Cross volunteers are working in three rest centres in Chorley, Salford and Bury. A spokeswoman said there were more than 50 people at the centre in Bury.
In York, people in the most-at-risk properties near the River Foss, a tributary of the River Ouse, have been advised by the Environment Agency to move valuable belongings upstairs and to prepare to be evacuated.
BBC north of England correspondent Danny Savage said on Saturday: "Everywhere I have been in Yorkshire today, whether people have lived there for 15 or 50 years, they have said the same thing to me - in their lifetime, they have never seen flooding like this."
BBC Weather forecaster Alex Deakin said the heavy rain would move southwards after midnight, leaving drier conditions in northern England.
While there would be more rain in the north during Sunday, it would not be as intense or prolonged as the downpours on Saturday, he added.
In other developments on Saturday:
- Two Met Office red warnings for rain were issued for parts of Lancashire and Yorkshire, but they have now expired
- There are seven severe flood warnings in Lancashire - meaning flooding is expected and there is a danger to life - and 24 in Yorkshire
- Houses in Mytholmroyd, West Yorkshire, and in Ribchester and Whalley, Lancashire, were evacuated after rivers burst their banks
- Electricity North West has warned power may not be restored to some homes until Monday. It said there were 8,100 properties across north-west England without power
- Soldiers, who were deployed to Cumbria on Christmas Eve, have been helping residents in Lancashire
- Every river in Lancashire exceeded record levels, the Environment Agency said
- There were diversions on the M62 after it was closed westbound between junction 20 at Rochdale and junction 19 at Middleton when a large hole appeared in the carriageway as result of rain
- A 200-year-old former pub located on a bridge over the River Irwell was destroyed by flooding at Summerseat, Greater Manchester
- The Scottish Environment Protection Agency has issued more than 10 flood warnings, in the Scottish borders and Tayside areas, as well as a number of flood alerts
- Emergency services including Lancashire Fire and Rescue Service and North Wales Police are warning motorists not to drive unless they need to, with some roads closed
Lancashire Fire and Rescue Service tweeted: "A severe flood warning means danger to life.
"It doesn't mean 'come and have a look'! Please don't come to visit Whalley or Ribchester now."
Ribchester resident Alan Tomlinson told the BBC the situation was "pretty grim" with many homes at risk of flooding.
"The village is under siege really," he said.
"The routes to the south are cut off and the route to the north, to Longridge, is under threat."
Kellie Hughes, a hairdresser who lives in Whalley, said the situation was "a million times worse" than a fortnight ago when floods also hit.
She said: "It's just horrific, really bad. I've got the sandbags down here and just doing the best I possibly can.
"There are no more sandbags anywhere. People are panicking. That's my business and my home, it's a double whammy."
The government's emergency committee, Cobra, met for a second day in a row.
Environment Secretary Elizabeth Truss, who announced a review of flood defences earlier this month, said the priority "continues to be protecting lives, protecting homes and protecting businesses".
Friends of the Earth climate campaigner Guy Shrubsole said the record rainfall and repeated flooding in December were a sign of changing climate and highlighted the need for greater action by the government.
Local Government Association environment spokesman Peter Box said councils were "pulling out all the stops" to find accommodation for those forced to leave their homes amid flooding.
"Storms in Cumbria and Lancashire have reminded us just how unforgiving and formidable nature can be but councils in the regions have worked hard to try and minimise the impact on residents," he added.
Flooding has brought travel "chaos" to roads in north Wales, with people rescued from cars and vehicles left stranded.
Capel Curig in north-west Wales had the most rainfall in the 24-hour period to 09:00 GMT Boxing Day, with a total of 168mm (6.6in).
Met Office forecasters had said up to 120mm (5in) of rain could fall over the most exposed sites in northern England on Boxing Day - with the average rainfall for the whole of December in the North West being 145mm (6in).
People can access information from council websites and the Environment Agency Floodline.
The agency is also operating a phone line - 0345 988 1188 - which will be staffed rather than offering recorded information.
Live flood warnings from the Environment Agency and the Scottish Environment Protection Agency.