The flooding crisis caused by devastating storms this winter is likely to get worse, despite forecasters predicting a welcome break in the weather for the coming week, David Cameron said.
The Prime Minister said while the weather was due to improve, the volume of rainfall over recent weeks meant groundwater levels would keep rising in many places.
His comments came after t wo people died on Friday - James Swinstead, an elderly passenger on a cruise ship in the English Channel, and minicab driver Julie Sillitoe, 49, whose car was hit by falling masonry in central London.
A 20-year-old pregnant woman and her unborn baby, from Tredegar, South Wales, also died in a crash on the A465 between Brynmawr and Garnlydan.
Large swathes of the UK remain on high alert as people battle to protect their homes and communities from the floodwaters.
The Ministry of Defence (MoD) said more than 3,000 servicemen and women were committed to helping the flood relief effort with "thousands more at a state of high readiness" to assist if requested.
The Environment Agency (EA) has 16 severe flood warnings in place for the South West and the Thames Valley, with almost 150 flood warnings and 230 flood alerts.
Homeowners stretched to breaking point by the relentless water will have some respite this week, as largely fine weather with lighter winds and less rain is expected for the the next five days.
Despite the forecast, the EA said parts of southern, south-west and central England remain at risk of flooding due to high river levels following the recent heavy rainfall.
Mr Cameron, who visited flood-hit Chertsey in Surrey before chairing the Government's Cobra emergency committee last night, said today would be "vital" as river levels were set to rise again.
"Thankfully, it does appear that we will see less rain and wind over the next few days," he said. "However, after so much rain over recent weeks, groundwater levels remain very high and in many places will continue to rise."
Paul Leinster, chief executive of the EA, said: " Despite an improving forecast, the risk of flooding will continue for many communities in southern parts of England over the next few days. We ask people to remain vigilant and take action where necessary.
"Environment Agency teams are working round the clock to support local authorities' relief effort. We have also teams out working to reduce the risk of flooding to communities and have deployed over 50 temporary defences."
Mr Leinster said more than 1.3 million properties have been protected since the start of December thanks to EA defences and the Thames Barrier, which was closed for a record 16th consecutive time yesterday.
Almost one million homes have been without power after downpours and high winds during the last week.
The Energy Networks Association said fewer than 16,000 homes remain without electricity after engineers restored power to 15,000 customers overnight, and more than 600,000 have been reconnected since the storms on Friday.
The severe storms have taken a major human toll in recent days.
Mother-of-three Mrs Sillitoe was killed close to Holborn Underground station after large chunks of masonry fell on to her silver Skoda Octavia on Friday night.
The minicab driver's passengers, a 25-year-old man and a 24-year-old woman, are being treated in hospital.
Mr Swinstead, 85, also died on Friday after the 22,000-tonne Marco Polo cruise ship was hit by a freak wave in the English Channel.
An injured woman in her 70s was also airlifted off the vessel, while 14 other people suffered minor injuries and were treated on board.
Emergency services and the Army rescued 32 people from the Marine Restaurant in Milford on Sea, Hampshire, at 10pm on Friday, evacuating them in an Army vehicle. Hampshire Police said there were no serious injuries.
A 20ft deep sinkhole also appeared yesterday morning under a quiet cul-de-sac in Hemel Hempstead, Hertfordshire.
The people living in 17 homes close to the site in Oatridge Gardens were evacuated, as the hole, measuring approximately 35ft wide, was investigated.
Carla Rance, 35, who lives yards from the sinkhole with her husband Ross, 30, and their three children, told the Sun on Sunday they had experienced problems for weeks.
She said: "I knew something was wrong. There has been subsidence and my porch collapsed.
"There has been a lot of movement and I have not felt safe in the house but when I reported it, I was told it was all fine. It is extremely upsetting."
Meanwhile, EA chairman Lord Smith admitted he "could have done better" during the flooding crisis and said the country needs to take a "serious look" at how it prepares for more extreme weather.
He told LBC Radio: "I think there are certainly some things that I could have done better.
"I think we could and should have worked harder to persuade partner organisations in Somerset to undertake some of the longer term work that's needed down there which we were wanting to start last year but we weren't able to get the other bits of money that we needed on to the table."
He added: " Now, we need to have a serious look as a country at how we prepare ourselves for that and how we build our flood defences."
There is increasing pressure on the Government to ensure greater care is taken on where new housing developments are built.
Mark Wilson, chief executive of insurance giant Aviva, said there should be a halt to building on "defenceless" flood plains.
He also called for houses to be given a "flood rating", telling the Sunday Telegraph: "As a nation we need to build more homes, but the cost of development must include the cost of defences. Let's be crystal clear: no defences, no development."
Lord Krebs, the climate change committee chairman, also called for action, the Sunday Times said, saying: "We are still slapping up homes and business properties in flood plain areas.
"We've adopted the build-and-defend approach in the past and we need an honest assessment. Is build and defend the right way, or do we say we will stop building homes in flood risk areas, we'll just build elsewhere?"
Meanwhile, a poll by ComRes for the Independent on Sunday and Sunday Mirror found that just 59% of people thought the Government was "beginning to get a grip" on the flooding.
Some 48% said the storms had made them more convinced that climate change was happening, compared to 30% who said their views had not changed.
An Opinium poll for the Observer found 51% thought Mr Cameron had responded badly to the floods.
Some 51% of those questioned said they believed issues around climate change and global warming caused the floods while 24% did not take that view, and 20% were neutral.