Farmland is to be transformed into “a paradise for wildlife” in a £12 million project to create a wetland environment of saltmarsh and mudflats.

The wildlife haven, which will be home to countless saltwater invertebrates, tens of thousands of birds and mammals, and millions of fish, will be formed by restoring 1,700 acres (700 hectares) of the Essex coast. It will also act as a natural flood defence.

Among the creatures that the reserve, being set up by a charity, is likely to host are otters and brown hares, snails and marine worms, and dozens of species of birds including redshank, greenshank, curlew, knot, dunlin and golden plovers. Plants will include sea asparagus.

Ornithologists are confident that the huge reserve will entice two varieties of bird back from the Continent to Britain to breed. The spoonbill has not bred successfully in Britain for 400 years and the Kentish plover vanished about 60 years ago. It is possible that the black-winged stilt, one of Britain’s rarest breeding birds, will also move to the Essex site.

Most of the prime farming land on Wallasea Island, close to Southend, will be returned to its natural condition as a tidal habitat. By 2020 the farmland currently used to grow peas for the Japanese snack market, as well as wheat and oilseed rape, will be fully established as a haven for wetland plants and animals.

The area is expected to become an important nursery for mullet, herring, bass and the fry of other fish and could help to restock the North Sea. Millions of fry could surge in and out of the tidal habitat, where they would find a ready supply of food.

The Wallasea Island Wild Coast Project will be announced today by the Royal Society for the Protection of Birds.

Mark Dixon, the project manager, said: “The area will be a supermarket for wildlife. The mud is a living soup.

“It’s not just birds that will benefit. Research has shown these areas are massively important for fish fry. The loss of fish nurseries could have as much impact on stocks as overfishing.”

In 2009 earthmovers are to be sent in to start landscaping the area and importing and spreading up to 15 million tonnes of mud. Breaches to the existing flood defence walls will be made in 2010 to let in the tide through pipes.

Construction of the reserve, which will include a visitor centre and at least eight miles of walks and cycle paths, will be done in two stages, the second section being complete in 2015.

Wildlife will begin returning to the land on the first tide to wash in and the transformation to fully established saltmarsh and mudflats will be complete about five years later.

The habitat creation project, the biggest in Britain and one of the biggest in Europe, is next door to a similar but much smaller scheme that was carried out by the Department for Environment, Food and Rural Affairs with the Environment Agency and finished last year.

Plants and animals have already moved into the Defra project area. Seals are among the recent arrivals, and otters had made themselves at home within six months of the last workman leaving the site.

The project, for which the RSPB is hoping for donations from businesses and individuals to help to cover the purchase and building costs, will replace sea walls as the flood defences, the Environment Agency having decided to stop trying to hold back the sea in the area with rigid structures.

The mudflats and saltmarshes will act as a vast sponge to sap the force of waves during storms. The landscapers have taken into account the rises in sea levels that climate change is expected to cause over the next century.

Graham Wynne, of the RSPB, said: “We will be restoring habitats that were lost more than 400 years ago and preparing the land for sea-level rises.”

Paul Woodcock, of the Environment Agency, said: “Such initiatives will help our estuaries and coastlines adapt to climate change and sea level rise for the benefit of people and wildlife.” Saltmarshes are one of the rarest habitat types in Britain, with less than 50,000 acres left.

On the waiting list

Invertebrates lugworm, harbour ragworm, marine snail, bush cricket, grasshopper, cockle, mussel, oyster, green shore crab

Fish flounder, eel, herring, mullet, bass, dab, sole, plaice

Birds redshank, greenshank, curlew, knot, dunlin, ringed plover, golden plover, teal, mallard, wigeon, brent geese, Kentish plover, spoonbill, black-winged stilt (below)

Mammals otter, brown hare, voles, seal

Plants sea asparagus, sea lavender, sea aster

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