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A big section of the Norfolk Broads as well as a cluster of villages and thousands of acres of farmland face being surrendered to the sea under secret plans to save the rest of the Norfolk coast from the impact of climate change.

A scheme drawn up by experts at Natural England, the body born out of the Countryside Agency and English Nature in 2006, envisages that 25 square miles of fen and fields would be wiped off the map for ever in an attempt to realign the coastline.

A leak of a draft paper that outlines the strategy has caused alarm throughout Norfolk.

With climate change likely to bring rising sea levels and coastal erosion, the cost of maintaining flood defences is considered too difficult and expensive. One scenario is to allow the sea to breach about 15 miles of the North Norfolk coast, between Horsey and Winterton, and flood inland for about five miles, as far as Potter Heigham and Stalham, to create a new bay. Hundreds of homes, about 2,500 acres of National Trust property and Hickling Broad would disappear under seawater before the end of the century.

Natural England confirmed yesterday that the plan was an option in a draft research paper. A report is to be given to the Government in September, setting out the threat of climate change and how it could be mitigated in the Broads, the Shropshire hills, the Dorset downs and Cranbourne Chase, and the Cumbrian high fells.

Details of the managed retreat of the coast were first discussed at a conference about the threat to the Broads in February. It is one of four options put forward for the Upper Thurne basin in the face of rising sea levels.

The paper said: “The broads (Martham, Horsey, Heigham Sound and Hickling) would become inundated by the sea, fen vegetation would be lost. It is likely over time that a spit would develop behind which coastal and intertidal habitats would develop.

“The increasingly unsustainable nature of the Horsey to Winterton frontage beyond the next 20 to 50 years opens up the possibility of realigning the coast as described within this time frame. It can also be argued that by selecting a radical option now, the right message about the scale and severity of the impacts of climate change is delivered to the public.”

Norman Lamb, Liberal Democrat MP for Norfolk North, said he was astonished that conservation chiefs had drawn up a plan that would leave a swath of his constituency under water. “The implications are pretty horrifying for the communities in that part of northeast Norfolk,” he said.

Martin George, a member of the Broads Society and an ecologist, said he was horrified by the proposal. “I would say about an eighth of an area we think of as the Broads would be lost, including Hickling Broad, the largest and most popular. Natural England have gone ahead with this report and it’s the first time a lot of people will realise their homes would be lost. Of course a plan like this would probably not be implemented for 25 to 30 years but the problem is the effect this will have on the value of their properties.”

The Broads Society wants cash to be spent on protecting beaches, but under the Natural England plan new sea walls would be built inland, easing pressure on other parts of the coast.

David Viner, principal specialist on climate change at Natural England, said: “If climate change goes on unabated then this is a possible scenario. It is responsible for us to address these challenges in the long term. One option is wholescale managed retreat.”

The Norfolk Broads, 74,000 acres north and east of Norwich, is one of Britain’s most popular holiday areas.

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