More than 300 flood alerts and warnings are in place across England, Scotland and Wales.
Lancashire and Yorkshire are bearing the brunt of flooding, with 15 severe warnings in place and homes evacuated, as heavy rain lashes parts of the UK.
The Met Office also issued two severe weather warnings, meaning danger to life, for rain in the two counties.
Several rivers have burst their banks, and homes in Ribchester and Whalley, Lancashire, have been evacuated.
Meanwhile, more than 300 flood alerts and warnings are in place across England, Scotland and Wales.
Almost a month's rainfall is likely to fall in one day in some places.
The Environment Agency said affected residents should "take action now".
Lancashire's severe flood warnings - meaning flooding is expected and there is a danger to life - are for two locations on the River Ribble at Ribchester, three on the River Calder at Whalley, as well as for the River Wyre at St Michael's North and St Michael's South.
The five Yorkshire flood warnings are around the Calder Valley area, with the River Calder, River Ryburn, Cragg Brook, Hebden Water and Walsden Water affected.
The two red Met Office warnings for rain have been issued for parts of Lancashire as well as for West Yorkshire and North Yorkshire.
They warn of "widespread flooding" leading to disruption, with possible danger to life.
Those being evacuated from their homes on Boxing Day have been told they can go to nearby Longridge Civic Hall.
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."
He said many residents had vowed to stay in their homes, moving upstairs if necessary - adding that the water in his cellar was already up to knee level.
Local Government Agency environment spokesman Peter Box said councils were "pulling out all the stops" to find accommodation for those made homeless by 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.
"Plans have also been put in place to ensure that older and more vulnerable people are not put at risk and can still access the council help they rely upon."
BBC weather said rain would continue to affect south and east Scotland on Boxing Day, as well as northern England and north Wales, but that the worst hit areas would be north-west Wales, Lancashire and west Yorkshire.
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).
Ben Lukey, of the Environment Agency, told BBC Breakfast: "We have been warning people to take action now to make sure that they keep themselves safe, to check out for vulnerable friends and relatives, to move valuable papers and other belongings to a safe place out of the way of water."
Met Office forecasters 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).
Floods Minister Rory Stewart told BBC Radio 4's Today programme: "As the rain falls, the rivers respond very quickly. Certainly what we've seen is rainfall levels that nobody's ever seen before."
On Christmas Day the Army was called in to support efforts to protect parts of Cumbria against further flooding and the government's emergency Cobra committee met to discuss the situation.
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.