There is already a small National Nature Reserve in the Wash, covering less than one square mile. This is being expanded tenfold to cover 35 square miles in the south-eastern corner, between the mouth of the river Nene and Snettisham.
The reserve, dedicated to the memory of the conservationist Sir Peter Scott, begins at the sea wall and stretches out over saltmarsh, sandbank and mudflat into the middle of the Wash. The Wash is one of the largest stopping points for millions of wildfowl and waders which migrate along the East Atlantic flyway, heading south in winter from breeding grounds in Scandinavia, Iceland and Siberia.
Some stay for a few days to rest and feed on the mudflats exposed at low tides; others remain for the winter. The Wash is rated an internationally important habitat for 15 bird species, including knot, the dark-bellied brent goose, shelduck and grey plover.
It is home for 6,000 common seals and harbours a tenth of Britain's saltmarsh, land covered in plants which can survive immersion in salt water during the highest tides. This habitat is being destroyed by development and sea level rise.
Yet today's declaration is largely symbolic; it gives the wildlife no extra legal protection. The area involved is already one of the Government's Sites of Special Scientific Interest and has a further measure of protection under an international wetlands treaty and a European Community directive.
Surprisingly, it is a nature reserve in which fishermen extract cockles and other shellfish using boats with suction dredging equipment, and where three wildfowling clubs shoot some of the migratory bird species. They are all authorised to do so by the Crown Estate Commissioners, which own the seabed and shoreline.
Now English Nature, the Government conservation arm, has negotiated its own 21-year lease with the commissioners to extend the National Nature Reserve. It has no objection to the activities of the fishermen and wildfowlers; the former control their catches through a fisheries committee and the latter provide voluntary wardens to patrol the area.
Bob Lord, site manager, said: 'They were there before we came on the scene and their use of the Wash's natural resources seems sustainable.' He is more worried about the growing disturbance from jet skis and powerboats.
English Nature would like to see better access to the reserve for walkers and birdwatchers. A public footpath covers only about half of the 12 miles of sea wall.