Boris Johnson has urged people to evacuate their homes if told to as Storm Christoph leaves thousands of properties at risk of flooding.
The Prime Minister chaired a Cobra crisis meeting on Wednesday after major incidents were declared in Greater Manchester, South Yorkshire and Cheshire.
Amber and yellow weather warnings are in force until Thursday for the storm, which is also threatening to bring up to 30cm of snow to northern areas.
Up to 3,000 properties in Greater Manchester could be affected by flooding, an assistant chief constable has said.
Meanwhile, train operator Northern advised customers in north-west England against travelling as flooding made it “increasingly difficult to operate a reliable service”.
Mr Johnson has urged people to heed the flood warnings and follow advice from authorities.
He told reporters: “There are some times where I have been to scenes where, alas, people have decided not to obey the advice and not evacuated.
“It is their right not to do so if they choose – it’s always people’s right to stay wherever they are.
“But it really is advisable – follow the advice. If you are told to leave your home then you should do so.”
Speaking at an online press briefing, Greater Manchester Police Assistant Chief Constable Nick Bailey said areas of Didsbury, Northenden and Sale near the River Mersey could potentially be affected by flooding, with a peak expected at 11pm on Wednesday.
He said: “The worst-case scenario, estimated by the Environment Agency, is that it could impact on up to 3,000 properties across all the areas I have mentioned to varying degrees.”
Downing Street has said Covid-secure facilities will be available for any people forced to evacuate as a result of the weather.
Mr Johnson said steps were being taken to ensure the transport and energy networks were prepared so that electricity outages would not be “severe” and that there were sufficient supplies of sandbags.
He added: “We are looking at a pattern of rainfall, possibly not so bad this week but worse possibly next week.”
More than 120mm of rain has already fallen in parts of the country, with 123.42mm at Capel Curig in North Wales in the 24 hours up to 2pm on Wednesday.
Crai Reservoir in South Wales saw the second highest total, with 115.6mm, and areas in Glamorgan and Cumbria also topped 100mm over the same period.
Some 100 flood warnings have been issued by the Environment Agency across England, with 209 less severe flood alerts, mainly across the Midlands and north of the country.
Almost the whole of England, Wales and Northern Ireland are subject to yellow weather warnings for rain until midday on Thursday, with a more serious amber warning stretching from the East Midlands to the Lake District.
The amber alert warns of the risk of flooding and deep floodwaters which could pose a risk to life, and there are further warnings for snow and ice in Scotland.
An amber warning for snow in parts of southern Scotland warned around 30cm could fall in areas above 400m, with up to 10cm likely to accumulate in lower regions until 8am on Thursday.
Train operator Northern said torrential rain and flooding across the region had led to the closure of several rail routes, while others had suffered “significant disruption”.
The company’s regional director, Chris Jackson, said: “Unfortunately the situation is only getting worse and we have had to make the difficult decision to ask our customers not to travel on the Northern rail network in the North West for the rest of today.”
Mr Jackson urged commuters wishing to use the service on Thursday morning to allow extra time for travel.
Met Office forecaster Grahame Madge described Storm Christoph as “quite a slow-moving system” which is bringing “a variety of weather” to the UK.
The meteorologist said: “While rain remains the main hazard in the south, further north we’ve got snow and ice remaining a risk.
“The system will work its way through, we are expecting significant totals of rainfall and when you combine that with snowmelt it can lead to localised flooding across the affected regions.”
There is a risk of further snow later in the week as Storm Christoph makes its way east, with accumulations expected in Scotland, northern England and parts of Northern Ireland, Mr Madge added.
Public Health England (PHE) has issued a cold weather alert for the North East, North West, and Yorkshire and the Humber from 9am on Thursday to Monday January 25.
Dr Owen Landeg, principal environmental public health scientist at PHE, urged people to “keep looking out” for frail or older neighbours and relatives, particularly those living alone or with a serious illness.
People were preparing for rising floodwaters in West Yorkshire on Wednesday morning, with towns in the Calder Valley laying out sandbags and flood wardens monitoring water levels.
Rail services have been disrupted on the Northern network after tracks flooded.
There are suspensions on services from Carlisle to Skipton or Maryport, all destinations from Rotherham Central, and between Manchester and Newton le Willows.
Rail services between Warrington Central and Liverpool Lime Street, Altrincham to Chester, and Wigan to Southport are also suspended.