The Met Office has warned that "all the evidence" suggests climate change played a role in the floods which have devastated thousands of homes following Storm Desmond.
Record amounts of rain have fallen in Cumbria, the worst-hit county, where many residents were evacuated and tens of thousands of properties left without power.
The Environment Agency said more than 5,200 homes and businesses were flooded, with Prime Minister David Cameron visiting the worst-hit areas.
The Met Office has issued a severe weather warning for rain this week as northern England, Scotland and Northern Ireland brace themselves for more heavy downpours.
Environment Secretary Liz Truss said there had been a "number of weather-related fatalities" and the severity of the weather was "unprecedented".
A body thought to be that of an elderly man was discovered in the swollen River Kent in Cumbria, and a 90-year-old man, Ernie Crouch, died after he was apparently blown into the side of a moving bus by strong winds near Finchley Central Tube station in London.
The Met Office said a new record had been set for rainfall over a 48-hour period, with 15.9in (405mm) falling in 38 hours at Thirlmere in Cumbria.
A 24-hour record was also recorded between Friday and Saturday evenings, with 13.4in (341mm) registered in Honister, Cumbria - more than a month's worth of rain in one day.
The Met Office's chief scientist Dame Julia Slingo said the extreme weather conditions were "extraordinary".
She told BBC Radio 4's World At One: "Is it to do with climate change? There can't yet be a definitive answer but we know that all the evidence from fundamental physics and what we understand about our weather patterns, that there is potentially a role."
Disruption caused by the storm looks set to continue, with many roads closed and the West Coast Mainline rail route to Scotland suspended with service unlikely to be restored before Wednesday at the earliest.
Electricity North West said 61,000 properties lost power across Heysham, Morecambe, Carnforth and Lancaster.
Meanwhile, Lancaster University confirmed it had cancelled all classes until the new year as a result of floods.
The Government said it will "look again" at flood defences amid mounting criticism that they failed to keep the deluge of water out of people's homes.
Speaking in Carlisle, Mr Cameron admitted the multimillion-pound barriers built following devastating floods in 2005 "weren't enough on this occasion".
Forty-six severe flood warnings were in place, meaning a danger to life, across the North West, along with dozens of less serious flood warnings and flood alerts over northern England and Wales.
Cumbria Police said up to 6,425 properties might have been flooded in the county but the most likely scenario was that 4,881 had been affected.
Some 2,685 properties in Cumbria remain without power, the force said.
In Carlisle, the Army was sent to help support emergency services evacuating people from their homes in streets where cars were almost entirely submerged.
Around 40 schools remained closed in Cumbria, while the disruption led to the cancellation of appointments and routine business across NHS hospitals and services.
Power has been restored to all homes in North Wales after heavy rain and wind left 700 without power on Saturday, and in Northern Ireland, major clean-up operations are under way in parts of counties Tyrone and Fermanagh after weekend flooding damaged homes and businesses.
An appeal by Cumbria Community Foundation to raise £1 million to support vulnerable individuals and families who have been badly affected by the floods is under way and has already raised more than £300,000.
Ms Truss told MPs that the Bellwin scheme allowing councils to recover the costs of tackling floods from Whitehall would be put into operation and further support schemes would be announced over coming days. The Government has already committed to a £2.3 billion programme of flood defences over six years.