People have been evacuated from their homes in Lancashire after the Met Office issued a red weather warning - danger to life - for the area.
A "take action" weather alert is in place for parts of the county and the Environment Agency has issued seven severe flood warnings for the North West.
Residents in Whalley and Ribchester were told to abandon their houses when flood waters poured through the streets after torrential downpours.
Forecasters said up to 4.7ins (120mm) of rain could fall in some areas already saturated by wave after wave of winter squalls.
The average rainfall for the whole of December in the North West is 5.7ins (145mm) - so not far off a month's rain could lash the region in hours.
The severe flood warnings - the EA's highest level of warning - are for two locations on the River Ribble at Ribchester, three locations on the River Calder at Whalley and two on the River Wyre at St Michaels.
People are advised to move valuables and take advice from emergency services about evacuation.
Around 120 flood alerts and 145 warnings were also issued by the EA, mainly for the North West, North East and Wales.
The Government's emergency Cobra committee met on Christmas Day while a company from the 2nd Battalion, Duke of Lancaster's Regiment, helped committees across the North West build miles of temporary flood defences.
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.
The Scottish Environment Protection Agency issued five flood alerts and 13 flood warnings.
Flood warnings extended across northern Britain, including the Calder in West Yorkshire and the Ribble, Wyre and Wenning in North Yorkshire, Wyre & Wenning. Pendle Water, Pimlico Brook, the Burrow and Hellifield Beck were also at risk of bursting their banks, the EA warned.
The Met Office expanded its amber warning of rain for Boxing Day, to encompass areas of Lancashire and western areas of both North and West Yorkshire.
A yellow warning of rain was issued for parts of north Wales, northern England and southern Scotland.
Floods Minister Rory Stewart said that 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.
"Certainly what we've seen is rainfall levels that nobody's ever seen before.
"If somebody had said two years ago when we were designing these flood defences that we could get 13 inches of rain in a day, the answer from the engineers would have been 'Why are you making that kind of prediction? We have never seen this before.'
"I think this is why people are right to start focusing on uncertainty and why people obviously are very interested in the question of climate change."
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.
Traffic Scotland urged caution over surface water on the roads in the Fife and Lothian and Borders regions.