Thousands of people have been flooded out of their homes or left without power after Storm Desmond wreaked havoc in parts of the UK.
In Cumbria and Lancashire more than 43,000 homes are suffering from power cuts, while an estimated 5,200 homes have been affected by flooding.
The weather has claimed two lives – in Cumbria and the Republic of Ireland.
David Cameron said the flood defences in Cumbria “were not enough” and some people might not be home for Christmas.
Storm Desmond lashed parts of Northern Ireland, north Wales, southern Scotland and especially north-west England over the weekend, and more heavy rain is expected this week.
Record-breaking amounts of rain fell in Cumbria, the worst-hit county – prompting the county to declare a major incident.
The devastation has resulted in criticism of the government after multimillion-pound defences built following floods in Cumbria in 2005 failed to keep the deluge out from people’s homes.
During a visit to Carlisle, the prime minister said the flooding was “absolutely horrific” and he promised to re-examine Cumbria’s flood defence plans.
In Cumbria, police said a man’s body had been recovered from a river.
And the body of a 70-year-old man from County Tyrone, in Northern Ireland, was found on a cross-border road in County Monaghan. It is understood his car got stuck in flooding and he was swept away when he got out of this car.
Mr Cameron, who chaired a meeting of the government’s Cobra emergency response committee on Monday morning, said the government would fully reimburse councils for the costs of dealing with flooding.
The immediate response had to be to find alternative accommodation for those who had been flooded out of their homes, and help with their insurance claims, he said.
“After every flood, the thing to do is sit down, look at the money you’re spending, look at what you’re building, look at what you’re planning to build in the future and ask, is it enough?” he said.
Emergency crews praised
Labour leader Jeremy Corbyn accused the government of failing to do enough to tackle flooding.
“Last year, the prime minister promised that ‘money is no object’ in dealing with flooding, itself a consequence of the destruction of our environment.
“But this has proved to be yet another false promise. In the last parliament, the government slashed spending on flood defences before the 2014 winter floods.”
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Media caption’Our estate has been flooded’
Making a statement in the House of Commons, Environment Secretary Liz Truss praised the work of the emergency crews, with some travelling from south Wales, Somerset and Lincolnshire to help.
She expressed her “deepest sympathy” to all those affected, and said there had been a number of weather-related deaths over the weekend.