The Government has warned of a “number of fatalities” caused by Storm Desmond, as the North West is hit by more rainfall.
Environment Secretary Liz Truss was speaking in the Commons after police said a body found in Cumbria is believed to be that of an elderly man who was reported missing amid reports he had fallen into a river.
The body was found in a stream running into the River Kent in the Staveley area of Kendal.
In Ireland, the body of pensioner Ivan Vaughan, 70, who was swept away in flood water, has been found in Co Monaghan.
Meanwhile, a teenager is fighting for her life after a group of four Dublin Scouts were swept into the sea in Wexford.
On Saturday, a man died after being blown into a bus by high winds in north London.
Sky News Weather Producer Chris England said Tuesday morning’s showers would be “concentrated in the North West, where they will be heavy and possibly thundery”.
:: The Latest UK Weather Forecast
Sixteen severe flood warnings – signalling a danger to life – are also in place across the North West, and the Met Office has issued a severe weather warning for rain later this week.
Across the north and in Scotland, roads have been destroyed, train passengers are being warned of a week of disruption and the Army is working to help people from their swamped homes.
The Government has said it will look again at flood defence spending in the wake of the deluge that has left thousands of homes in Cumbria and Northumberland flooded.
:: Cameron Says Flooding ‘Absolutely Horrific’
One rain gauge in Cumbria recorded 341mm of rain in 24 hours over the weekend – beating the previous highest-ever recorded English rainfall of 316mm in 2009 – much of which has poured into rivers and towns further downstream.
Questions have been asked about why multimillion-pound flood defences, which were upgraded in 2010 to withstand a “once in 100 years” flood, were breached at the weekend.
Mr Cameron has visited the region, meeting a flood-hit homeowner and thanking emergency services for their efforts.
The PM was shown inside the home of mother-of-three Lesa Boyko, whose home in Carlisle was hit by two feet of flood water after the River Eden broke through flood defences.
Mr Cameron said the Environment Agency would carry out a study to assess the flood barriers and to learn from the widespread flooding, following similar events in 2009 and 2005.
“The emergency services have been brilliant but that’s no consolation to people who you know, face a very wet few days and then not perhaps being home for Christmas,” Mr Cameron said.
“But after every flood the thing to do is sit down, look at the money you are spending, look at what you are building, look at what you are planning to build in the future and ask, ‘Is it enough?’
“And that’s exactly what we will do.”
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Liz Truss, the Environment Secretary, earlier told Sky News: “It delayed the floods so it gave us an opportunity to protect people and evacuate people.
“Of course we will learn the lessons that we can from this unprecedented event.”
Hundreds of people are facing Christmas away from home or trying to rebuild their lives as their sodden houses dry out.
Cumbria Police said up to 6,425 properties could have been flooded, but the most likely scenario was that 4,881 have been affected.
:: Cumbrian Grit In Face Of Devastating Floods
People are being given bottled water to drink because treatment works have been flooded and there are reports that sewage has been forced on to the streets.
In Cumbria, the worst hit county, several bridges and roads have been washed away and many others declared unsafe for the meantime while checks take place.Rescue teams involving the local Duke of Lancaster’s Regiment and the RNLI helped to evacuate people as thousands of shops, homes and businesses were inundated and dozens of schools were closed.