The number of severe flood warnings has doubled in the last hour as hundreds of residents were evacuated from their homes.
There are currently more than 335 alerts in place across England, Wales and Scotland, including 15 at the most severe level.
Lancashire and Yorkshire have been hit by downpours, with people in a number of towns and villages forced to leave their homes after being stranded by rising waters.
The Met Office issued two of its most serious red weather warnings - danger to life - for the area.
Residents in Whalley and Ribchester in Lancashire were forced to abandon their homes when flood waters poured through the streets after torrential downpours.
Todmorden in West Yorkshire has also been hit and the waters are continuing to rise as the rain keeps falling.
Lee Fraser, who lives on Halifax Road, said the road between the town and the neighbouring village of Hebden is submerged.
"It's getting worse and worse, it's been raining really heavily since last night," he added.
"The siren went off at about 7am this morning and 10 minutes later everything started flooding.
"A lot of people are moving their stuff upstairs in their houses and the police came and closed the roads.
"It's absolutely tipping it down, so it's only going to get worse by the look of it."
Lancashire Fire and Rescue Service warned people to stay away from the Whalley and Ribchester areas and told motorists not to drive unless they have to.
"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," the service said in a tweet.
Flood sirens sounded at Walsden, Todmorden, Hebden Bridge and Mytholmroyd in Calderdale, Yorkshire.
Abbi Blackburn has been left stranded in her home in Walsden after around five feet of water poured into her cellar.
"It's getting quite bad and it's still walloping down with rain," she said.
"We didn't have any sandbags, it started at about 6am this morning.
"We've lost two freezers, my washer and dryer. It's at least five-foot deep down there.
"The Environment Agency rang up and said something about evacuation but we're not leaving, we're staying put.
"It's coming down, coming up and coming in.
"The road is totally submerged, it's been like that for hours.
"You'd be taking a bloody risk walking, it's over the top of wellies and it's too deep to drive through."
The EA has issued 87 warnings and 27 alerts for flooding in Yorkshire on Saturday and Sunday.
Alison Baptiste, EA flood duty manager, said: "Our thoughts are with all those who've been coping with serious flooding to their homes and businesses repeatedly over Christmas and those who face the risk of further flooding.
"On Boxing Day we have issued several severe flood warnings meaning a risk to life, we urge people to check their flood risk, prepare for flooding, follow advice from emergency services and never to risk driving through flood water."
A number of Boxing Day football and racing events have been called off , including the Welsh National in Chepstow and Blackburn Rovers fixture.
The match between Blackpool and Oldham was postponed, as were the games between Hartlepool and Notts County, Morecambe and Mansfield and Accrington and Carlisle.
Officials at Wetherby races cancelled both days of their Christmas card and Saturday afternoon's meeting at Limerick has also been abandoned.
The Government's emergency Cobra committee met on Christmas Day while a company from the 2nd Battalion, Duke of Lancaster's Regiment, helped communities across the North West build miles of temporary flood defences.
Lieutenant Colonel Hamish Cormack and his troops were deployed to Cumbria on Christmas Eve and are now in Lancashire helping evacuate residents.
"A lot of us are from the north west and for them to come out here and do this sort of thing, come face-to-face and help people has been humbling," he told Sky News.
"The guys just want to get in there and do as much as they can, they don't care about sleep they don't care about anything when they're helping people like they are here."
The EA said 85% of the country's temporary flood barriers had been sent to Cumbria, where rainfall has smashed records, and the Lakeland region braced itself again ahead of the deluge.
Lancashire has been hit by the downpours that were forecast to fall in Cumbria, Lancashire Police said.
"Lancashire is experiencing the rainfall expected to fall in Cumbria and a further 50mm to 80mm may fall in the next six to nine hours," the force tweeted.
"The band of rain currently over Lancashire is expected to move northwards in the afternoon before returning in the early to mid-evening."
The Scottish Environment Protection Agency issued seven flood alerts and 16 flood warnings.
The Met Office issued a yellow warning of rain for parts of north Wales, northern England and southern Scotland.
Floods Minister Rory Stewart said rainfall levels in the flood-hit areas were unprecedented.
"We're looking potentially again today at maybe a month's rainfall coming in a day. That's falling on ground that's very saturated. As the rain falls, the rivers respond very quickly," he told the BBC Radio 4 Today programme.
In the Outer Hebrides, CalMac ferry sailings were cancelled for the day in North Uist and South Uist. Services have also been affected on Bute and the Isle of Lewis.
Peter Box, environment spokesman for the Local Government Association, said councils are "pulling out all the stops" to find accommodation for people made homeless by the 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.
"We also ask that residents check in on frail and vulnerable neighbours to make sure that their homes haven't been damaged and that they are warm and comfortable."