By Tim Hirsch BBC environment correspondent

The government has been warned in no uncertain terms that Britain faces huge extra costs from flood damage and coastal erosion unless policies to protect vulnerable communities are stepped up in coming years.

Flooding could become an even more common occurrence

This warning comes not from some lobby group, but from a team of experts headed by Tony Blair's chief scientific adviser.

The Foresight Flood and Coastal Defence Project was charged with looking ahead at the possible impact of climate change over the next century, and the extra risks this might bring through heavier rainfall, more frequent storms and rising sea levels.

Its conclusions are sobering. Under the most extreme scenario for global warming, it suggests that the total damage from coastal erosion and river flooding could leap from �1bn now to around �20bn by 2080.

And even if the world succeeds in curbing the greenhouse gas emissions linked to climate change, it concludes that current policies will be inadequate to prevent these costs from rising substantially.

While recognising the extra money earmarked for flood defences in the past two years, the panel says that this increase must continue steadily in coming decades if current levels of protection are to be maintained.

'Tough dilemma'

This is not boating for pleasure

The report identifies a tough dilemma facing the government. There's a huge demand for new housing, especially in south-east England, and if greenfield sites are to be avoided, the land available for development is often in the very areas most at risk from flooding.

So the needs of the economy and social provision have to be balanced against the danger of creating new problems for the future by allowing further development on flood plains.

The Environment Agency warns that even after stricter government guidance aimed at avoiding the mistakes of recent decades, too many bad planning decisions are still being made.

The agency's chairman Sir John Harman welcomed the report, saying: "It is vital that we reduce the extent of inappropriate building in flood-prone areas, otherwise we will just store up problems for the future.

"Planning authorities must become more flood-aware in their decisions."

Some of the most frightening forecasts in the report centre on the increasing pace of coastal erosion expected to accompany climate change, as sea levels continue to rise and low-lying shores become more vulnerable to storm surges.

Coastlines at risk

It estimates that on current spending plans, about a third of existing coastal defences could be destroyed, putting large parts of the east coast at risk.

Which leads to another dilemma for the future. How far will the government continue to play Canute and hold back the sea, and where might the coastline be redrawn?

There will be increasing pressure to prioritise protection on large population centres or coastal industry - isolated communities at risk of inundation could find it even more difficult to get sea defences renewed.

Some like the small village of Happisburgh in Norfolk are already seeing the impacts - 26 houses have had to be abandoned as erosion takes large chunks out of the sand cliffs on which they were built, and a fierce local campaign for new defences has been unsuccessful.

The Environment Minister Elliott Morley says it is impossible to protect every inch of coastline.