John Gummer, the Tory former secretary of state for the environment, said there should be an urgent new survey of the areas at risk. Speaking at the launch of a World Wide Fund for Nature report, he predicted that 13,000 hectares of shoreline along the East coast of England - an area more than the size of Jersey - would be lost to the rising seas in the next 20 years.
Mudflats, salt marshes and coastal lagoons, all of immense value to wildlife, are the areas that will disappear first under the sea, says the report, written in conjunction with the Wildlife Trusts. It says the process is already visible in Essex.
"Tides that reach five millimetres higher each year have destroyed a quarter of the salt marshes of Essex - more than 15 square kilometres - in the past 15 years," says the report. "Typical rates of retreat are two metres per year in estuaries and four metres in more exposed coasts. Now they are threatening the sea walls that protect the farmland behind the marshes."
The process is particularly severe in South-east England because Britain is "tilting" geologically - the South-east is sinking while the North- west is slowly rising. A current sea-level rise of two millimetres a year is being compounded by a six millimetre sinkage rate.
But the rate of sea-level rise is likely to increase sharply: in 20 years, according to WWF, the sea will be about 12 centimetres higher than now in South-east England, but in 50 years it is expected to be 50 centimetres higher - more than 18 inches.
The global rise in sea levels themselves is perhaps the most immediately damaging consequence predicted of climate change, putting at severe risk hundreds of low-lying countries such as Bangladesh.
Although melting glaciers will add to the sea's volume, levels will rise principally because the seawater will expand as it warms. United Nations experts expect a world average rise of between 38 and 55 centimetres by 2100.
Such estimates have hitherto given the idea that it is a phenomenon far in the future but yesterday Mr Gummer, whose Suffolk constituency includes 74 miles of coastline, was at pains to stress that it has already arrived. "People haven't woken up to the fact that sea-level rise is happening in Britain now," he said. "It is not something that is going to threaten our children. It is threatening us."
It was already visible in many places along the coast, he said. "Just look under the pier at Felixstowe, and you can see all its stanchions, where the beach has disappeared. You only have to walk along the sea defences at Aldeburgh, which I opened five years ago, and see where the sea is now."
Mr Gummer said there were dangers in the Ministry of Agriculture's new policy of "managed retreat" - abandoning sea walls and other such man- made defences in favour of letting natural habitats such as salt marshes absorb and slow down the sea's incursion.
"If officially nothing is to be done, the temptation will be to cut the funding. This is not a party political point. All governments do it. But we need a lot more research on what managed retreat means and how it works. People who live near the sea want to defend their communities.
"We need to have a proper new survey of the whole of the East coast, and then apply the principles of managed retreat to that, to see how best we can use the resources which ought to be available."
As environment secretary, Mr Gummer was heavily involved in preparations for last year's Kyoto summit conference at which countries of the developed world agreed to cut their greenhouse gas emissions. The follow-up meeting takes place in Buenos Aires next month, and he stressed its importance by making the point: "What happens in Buenos Aires next month affects Aldeburgh and Felixstowe."