A 20-mile-long sea wall has stopped the ebb and flow of the tides that have made the Saemangeum wetland one of the most important feeding points in the world for migratory birds. British conservationists from the Royal Society for the Protection of Birds (RSPB) said yesterday that since the wall was closeda year ago the wetlands have been turned from a thriving bird reserve to a barren wasteland.

"The upper tidal flats are now more like a desert," said Sarah Dawkins of the RSPB. "You can drive out on to the estuary for several miles. The birds are confined to the water flowing from the two main rivers draining into the area."

"Very quickly they are depleting food resources. One week they are there, and the next they are gone."

The Saemangeum wetland - one of the biggest in the world - was once an important refuelling area for around 400,000 migratory birds making the 15,000-mile round trip between the southern hemisphere and south-east Asia and their breeding sites in Russia and Alaska. On any one day there could be more than 150,000 birds from 25 species feeding on the mudflats. Two species in particular, the spoon-billed sandpiper and Nordmann's greenshank, face extinction because of their reliance on Saemangeum and the other remaining tidal flats of the Yellow Sea,the RSPB warned.

"Estuaries should be fantastic places, full of the bustle of shorebirds feeding on shellfish and worms in the mud and sand," Ms Dawkins said. " The wall has blocked the life-giving ebb and flow of the sea, boats are stranded waiting for the tide that will never come and the mudflats are strewn with mile upon mile of litter. Saemangeum really was the jewel in the crown yet the place is dying."

The South Korean authorities began to build the sea wall 15 years ago to drain the wetlands for agricultural purposes - they wanted to establish rice paddies in the area. But it has emerged that there is not enough fresh water for growing crops and there are problems with pollutants in the drained estuary.

Ms Dawkins said: "Now they are talking about building a golf course, a huge casino or even a Formula 1 track.

"It would be like putting a casino in The Wash."

Before the sea wall was built, the difference in height between high tide and low tide at Saemangeum was 23ft. Now it is less than 7in - well short of the flow needed to replenish the estuary.

Despite the huge changes incurred as a result of the seawall, there is still a glimmer of hope for wildlife if the authorities can be persuaded to keep the sluice gates in the sea wall open to allow some limited tidal flow, said Nial Moores, director of the charity Birds Korea.

"International appeals to the authorities here in South Korea would underline just how precious Saemangeum is," he said.

"The Ministry of Agriculture claims the birds will just move to neighbouring estuaries but the birds there are already fighting over food and at least one of these estuaries may also be reclaimed." If the Saemangeum wetlands can be saved, they could become a magnet for eco-tourists from all over the world, which would help sustain the local economy, said Park Meena of Birds Korea.

"The birds are still coming and parts of the site are still alive so there is a chance we can save it," Ms Meena said."If the sluice gates were opened the tides would return, restoring life to the mudflats and bringing food both to the birds and the people with whom they co-exist."