The combination of a severe storm and a high tide would cause catastrophic damage to parts of the UK, leading researchers have warned.
Carlisle rescue teams at work in 2005
The scientists have criticised government cuts to national funding for flood defences, saying firm commitment is needed to avert disaster.
They named Hull, Portsmouth and the Thames Gateway as areas at high risk.
The warning comes in advance of two high tides forecast to occur later this Summer.
Severe storms and high tides
Speaking at a press conference in London, the researchers said that low-lying areas such as Hull, where one-quarter of the population lives on the flood plain, would suffer badly in the event of a severe weather combination.
"We're not going to get a hurricane on the scale of Katrina. But that's not to say we're not going to get extreme weather events, for example a surge coming down the North Sea," warned Dr Jean Venables, former chairman of the Regional Flood Defence Committee of the Environment Agency Thames region.
"We have to accept that no matter how much money we spend, there will always be a bigger event than what we've designed for, and the consequences, tragically, are often death."
In London, extensive flooding could be induced by rain alone, says Edmund Penning-Rowsell, head of the Flood Hazard Research Centre at Middlesex University.
Very intense summer thunderstorms could cause drains to fail, leading water levels to rise with bewildering speed.
Given the city's high proportion of basement accommodation, significant loss of life could result.
An avoidable tragedy
The scientists are confident that the risks can be managed, given the proper investment. The decision to build in low-lying places must be accompanied by careful, thorough planning to avoid devastating losses.
But they are concerned that the Department for Environment, Food and Rural Affairs' (Defra) move to cut flood defence budgets, to make up for losses from the failures in the new farming subsidy system, signal that the government's previously "positive" management approach is under threat.
Severe floods submerged the roads of Callander, Scotland, in 2005
"We see self-inflicted hazard in this country, just as we see self-inflicted hazard in other parts of the world," says Professor Penning-Rowsell.
"Risk is not an act of god; it's an act of humans. Flood defence planning should not be the subject of political machinations and varying budgets."
A rising coastal population
Despite the increasing risks, the world's coastal population is continuing to grow at double the mean global rate.
Housebuyers may not be fully aware of the risks that their properties are under, and developers ignore the problems associated with building in a flood plain, the researchers claim.
"I wouldn't buy a house in Thamesmead. Not because the risk [of a severe flood] is very high, but because the consequences of a flood in Thamesmead would be catastrophic," says Professor Penning-Rowsell.
Flood risk evaluations have recently been dropped from housebuyer information packs.