Too many people ignore the risks of flooding despite its increasing prevalence at home and abroad, the Environment Agency has warned.
Five million people in England and Wales live in flood risk areas
Five million people in two million properties in England and Wales live in flood risk areas.
Despite this, up to two-fifths (41%) are unaware of the threat, an agency survey suggests.
A sample of 1,022 people living in flood risk areas across England and Wales were questioned.
"Although we're unlikely to see flooding in the UK like that caused by the Boxing Day tsunami and Hurricane Katrina, there is still a significant flood threat here from extreme rainfall and coastal surges," chief executive Baroness Young said.
"Devastating floods do happen here too, like those in autumn 2000 and more recently in Boscastle and Carlisle."
The floods of five years ago, during Britain's wettest autumn for 270 years, saw whole villages evacuated as more then 700 areas were battered by storms.
As many as 100,000 homes were damaged in some of the worst floods ever to hit the UK with a financial cost of more than £1.3bn.
In August 2004, much of the Cornish village of Boscastle was destroyed by a flash flood with residents being rescued from rooftops as cars were swept into the sea.
And in January this year, three people died and hundreds had to be rescued in flood-ravaged Carlisle.
"There's a tendency for people to think 'it'll never happen to me'," Baroness Young said.
"The fact is, it could, we just don't know when.
"People in this country cannot afford to be complacent about flood risk."
On the fifth anniversary of the October 2000 floods, the Environment Agency is launching an advertising campaign urging the public to find out if they live or work in areas most at risk from flooding.
More than two-thirds (69%) of those surveyed by the agency said they were unlikely to take action to protect themselves and their property despite living in a flood risk area.
The research also suggests that only 7% of people living in flood risk areas know how to get flood warnings and that one-third (34%) do not know if their buildings and content insurance covered flood damage.
And two-fifths (42%) admitted they would not know what to do if their property became flooded.
"We're urging people to try to reduce the massive financial and emotional costs of flooding," said Baroness Young.
"So the message is be prepared for flooding."