Cuckmere Haven in East Sussex is the quintessential English beauty spot. In the bed of the valley a river meanders lazily down to the sea. Cows graze in lush green pastures. The smooth flanks of the South Downs rise on either side until those to the east terminate abruptly in the sheer chalk cliffs of the Seven Sisters with their panoramic views of the English Channel.

This idyllic scenery is enjoyed by more than 400,000 visitors a year. A beautiful photograph of a 19th-century coastguard cottage on the valley’s westward side sustained Robbie Turner through the horrors of the Second World War in the film Atonement. But Cuckmere Haven is also — though the casual hiker would never guess it — the centre of a raging controversy over government plans to let it revert to what it was before man began tinkering with nature: a tidal estuary with saltmarsh and mud flats.

The Environment Agency spends up to £50,000 a year on keeping the river mouth clear, on maintaining great shingle barriers on either side of it so that the sea cannot roll in, and on repairing the deteriorating banks of a channel the Victorians cut through the valley in the 1840s to aid navigation and stop the river flooding.

The agency reckons that to keep the valley as it is would cost £18 million over the next half century — money it badly needs for the protection of more populated coastal areas against rising sea levels and increasingly frequent storms induced by global warming.

The time has come to “work with nature, not against it”, says Peter Midgley, the agency’s Kent and East Sussex manager. The plan is to end maintenance work gradually so the sea can flood the valley at high tide once again. Mr Midgley, 57, positively relishes the prospect. There will be more birds, more marine life and much greater biodiversity, he says. “It will be even more lovely. It will be more natural. At the moment it’s a completely man-made, man-maintained landscape.”

Others are appalled. Thousands have signed a petition to save the Cuckmere valley. Nigel Newton, the chief executive of Bloomsbury Publishing in London, who owns one of the coastguard cottages, sees it becoming a dreary mud flat resembling Passchendaele, the First World War battlefield. “I defy you to name a more beautiful place in the whole of England, but this school-geography-project-gone- wrong will wipe it out,” he says.

The row over the Cuckmere valley is not a one-off. It is a harbinger of things to come right round England’s low-lying eastern and southern coastline, from the Humber to the Solent, as the impact of global warming becomes increasingly pronounced.

Thousands of acres of coastal land have been surrendered to the sea already, and this process of “managed retreat” or “managed realignment” is gaining momentum as the Government accepts, in effect, that Britannia no longer rules the waves. “However good we are at engineering, nature is more powerful than us,” says Mr Midgley. “We can slow its effects but, at the end of the day, nature is going to win.”

Sea levels are rising by 2mm a year, a rate projected to increase to about 15mm by the end of the century. That rise is exacerbated by a tectonic tilt that is depressing southern Britain as northern Britain rebounds from the Ice Age. Climate change means storms will batter coasts more often.

In the face of such threats the Government is increasing sharply its spending on schemes to counter floods and coastal erosion. It has earmarked £2.15 billion for the next three years, though much of that will be spent on inland schemes. But, like Canute, it realises that it can no longer defend the entire coastline and must give priority to the protection of more populated areas.

That means “some very difficult decisions” will be required, Hilary Benn, the Environment Secretary, admitted recently. The Environment Agency, which took over prime responsibility for 6,000 miles of coast and 800 miles of coastal defences from 90 different authorities last month, had to study every stretch of shoreline and say “here are areas we really do need to protect and others which may be difficult to protect for ever from rising sea levels”, Mr Benn said.

Farmers and property owners are appalled at the prospect of becoming the country’s first climate-change refugees. There is, for example, an uproar in Norfolk over a suggestion by Natural England, the Government’s conservation advisory body, that several miles of coastal defences between Eccles and Winterton are unsustainable, and that the sea should be allowed to flood 25sq miles (65sq km) of the Norfolk Broads including six villages, hundreds of homes and thousands of acres of prime farming land. “It would be a tragedy to lose a wonderful area of the country by allowing the sea in without a fight,” declared General Sir Richard Dannatt, head of the British Army and a Norfolk landowner.

There is another outcry farther down the coast where the Suffolk town of Southwold and the neighbouring village of Walberswick are bitterly resisting agency plans to stop maintaining flood defences along the Blyth river estuary that separates them. The agency says that the plan would save £34 million over several decades. Locals say it will destroy 40 homes and hundreds of acres of farmland, and have delivered a 1,500-signature petition to Mr Benn wearing Churchill masks invoking the wartime leader’s “fight them on the beaches” spirit. “We have been defending this coastline for thousands of years and this is the first government to decide that we will give in,” John Gummer, the former Environment Minister and Conservative MP for Suffolk Coastal, complained recently to another newspaper.

A new strategy for preventing the Humber estuary from flooding is also causing concern. It envisages 1,500 acres of farmland reverting to wetland in the coming decades, threatening several villages and 2,000 properties. The upside would be enhanced protection for 400,000 people, including the entire population of Hull, living on land that is at present liable to flooding. What compounds the property owners’ anger is that the Department for Environment, Food and Rural Affairs offers no compensation where sea defences are allowed to lapse.

But while farmers fume, and ask why the British do not defend their coastline like the Dutch, environmentalists are thrilled that thousands of acres of marshes, drained for agriculture in previous centuries, are being restored to their original state.

In the Blackwater estuary in Essex, 200 acres (80 ha) of land at Abbotts Hall Farm were flooded deliberately in 2002 when five big holes were punched in walls that had held the sea at bay for the previous 300 years. Maintaining those walls would have cost £500,000 over the next 20 years.

Today the area is a thriving salt marsh teeming with redshanks, wigeon, egrets and teal, its waters a nursery for herring, dover sole and sea bass. “The ecological success is massive,” says John Hall, director of the Essex Wildlife Trust.

In the 14th century the Essex coast was bordered by about 100,000 acres of marshland. That has been whittled away to less than 2,500, but finally the process is being reversed. The Abbotts Hall experiment — the biggest in Europe at the time — has since been replicated at numerous sites in the Blackwater, Crouch and Humber estuaries, and in the Wash.

The Environment Agency calls these new marshlands “soft defences” and sings their praises. They are less ugly and much cheaper to maintain than “hard defences” — concrete walls and shingle barriers that would have to be built higher and stronger. They absorb huge amounts of water, making floods less likely. They also break the power of incoming tides, absorbing almost all the energy from storm waves before they hit dry land.

“It is a real win-win,” says Alison Baptiste, the Environment Agency’s grandly named Making Space for Water Co-ordinator.

Back in Cuckmere Haven the argument rages on. Tony Whitbread, chief executive of the Sussex Wildlife Trust, enthuses about a scheme that will, he believes, enhance the valley.

Mr Newton disagrees vigorously. He accuses the Environment Agency of destroying beautiful habitats in the guise of returning them to nature merely to save money.

“Managed retreat is embracing defeat,” he says.

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