The Army has been called in to support efforts to protect parts of Cumbria amid fears of further flooding.
Troops have been helping put out sandbags in preparation and flood defence gates have been closed in Cockermouth, Carlisle and Keswick.
There are five severe flood warnings and more than 70 flood warnings in place, mostly in northern England.
Cumbria has seen severe flooding three times this month. Heavy rain is feeding swollen rivers and saturated ground.
More than 110 flood alerts - signifying flooding is possible, be prepared - have also been issued.
The Scottish Environment Protection Agency has issued five flood alerts, including Tayside, where there are also eight higher-level flood warnings.
'Most at risk'
Rain was expected to spread across northern England and push northwards into Scotland through the course of Christmas Day.
On Boxing Day, rain will continue over much of Scotland, northern England and north Wales.
Forecasters warn the heaviest and most persistent rain is likely to be over north-west Wales and north-west England - 60 to 80mm (2in-3in) is expected, possibly 120mm (5in) over hills.
The Met Office has issued an amber warning for rain - meaning "be prepared" - for Cumbria on Saturday, and also for parts of Lancashire.
An Environment Agency spokesman said: "Cumbria is the area most at risk from further river and surface water flooding later today and tonight, with parts of Lancashire and Yorkshire also at risk.
"There is also the potential for some flooding along parts of the River Severn in Shropshire, Telford and Wrekin and Worcestershire over the weekend and into Monday."
Cumbria Police is urging motorists to drive only if "absolutely necessary" tonight because of heavy rain.
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.
The Ministry of Defence said one company from 2nd Battalion Duke of Lancaster's Regiment (2LANCS), based at Weeton Barracks near Preston, had been deployed to help build flood defences in Warwick Bridge, Braithwaite and Appleby, early on Friday morning.
As well as the Army, the Environment Agency has 700 staff on stand-by to help people.
It has transported over two kilometres of temporary flood barriers and more than 20 extra pumps to the north of England and teams of workers have been out checking and maintaining flood defences, clearing blockages in watercourses and monitoring water levels.
'We got donated a Christmas dinner'
Eight centres opened across Cumbria to put on Christmas lunch for those who have been flooded out of their homes or do not have the facilities to cook.
Residents also described receiving donations to help them enjoy the holiday.
Morag Little, whose house in Carlisle has been flooded twice, said despite the problems she and her family had faced, they would still be enjoying Christmas dinner together.
"We got donated a full Christmas dinner... we've got the turkey, the Brussels sprouts, we've got the lot."
Cumbria has already recorded the wettest December since records began in 1910.
Thousands of properties were affected when Storm Desmond brought a record amount of rainfall to Cumbria during the weekend of 5-6 December.
Four days later, further rain led to more flooding, and some parts of the county were inundated for a third time in a month on Tuesday after another deluge.
About 40 bridges and roads remain damaged or closed in the area.
Defence Secretary Michael Fallon said: "Even at Christmas our Armed Forces are keeping us safe. Once again they are responding to the Cumbria floods with a level of commitment that is to be applauded."
Lt Peter Coates, who was among a group of soldiers working in Braithwaite near Keswick, said: "People have been really happy to see us. They've given us little snacks. They're just glad we're getting on with the task.
"If it does flood people need to be prepared. Peoples' lives potentially, as well as their property, can be at risk so whatever we can do to help we will do to help."
Environment Secretary Liz Truss, who chaired a conference call of the government's emergency committee Cobra on Friday, said: "Our priority continues to be protecting lives, protecting homes and protecting businesses.
"I would like to pay tribute to the tireless work of front-line staff over the last month and the resilience of those communities affected, which I have seen first-hand.
"This has been a very difficult time for all involved, especially people flooded out of their homes at Christmas for which I have enormous sympathy."
Cluanie Inn in Inverness was where the most rain fell overnight from Christmas Eve to Christmas Day, with 19.8mm (0.78in) recorded in the 12-hour period to 05:00 GMT, the Met Office said.
This follows strong winds brought by Storm Eva to north-western parts of the UK in the early hours of Christmas Eve morning.
However, its impact was more keenly felt in Ireland where more than 2,000 homes and businesses were left without electricity on Christmas Eve.
Live flood warnings from the Environment Agency and the Scottish Environment Protection Agency.