Thousands of properties flooded as northern regions feel devastating impact
The deaths of a "number" of people have occurred in incidents "caused or exacerbated" by Storm Desmond in Britain, the British Environment Secretary has said.
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.
On Saturday, a man died after being blown into a bus by high winds in London.
Sixteen severe flood warnings - signalling a danger to life - are in place across the North West of England and the Met Office has issued a severe weather warning for rain later this week as northern England, Scotland and Northern Ireland prepare for more heavy rainfall.
Across the north of the country 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.
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 multi-million-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.
Mr Cameron 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.
He 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."
Ms Truss 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.
Some 2,685 properties also remain without power, the force said.
Electricity North West said 61,000 customers had lost power across Heysham, Morecambe, Carnforth and Lancaster. 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 have helped to evacuate people as thousands of shops, homes and businesses were inundated and dozens of schools were closed.