As a result, they said, the Coto Donana reserve, home to huge populations of waterfowl, ducks and wading birds, is now facing a further threat of major pollution and wildlife mortality.
Millions of tonnes of toxic sludge which has still not been cleaned up will be washed over its marshes and mudflats when the autumn rains arrive.
This pollution is likely when the bird population on the reserve in south-western Spain more than doubles with the arrival of hundreds of thousands of winter visitors from northern Europe such as teal, black- tailed godwits and greylag geese, many from Britain.
The sludge, containing high levels of heavy metals such as arsenic and cadmium, came from a giant iron mine waste reservoir which burst 30 miles upstream from the Donana at the end of April.
It cascaded down the River Guadiamar, covering farmlands, olive groves and rice paddies with a thick carpet of toxic mud, and badly damaged the marshes which are the ecological core of the area, but was stopped just short of the Donana National Park itself.
More than 25,000 kilos of dead fish were collected and nearly 2,000 adult birds, chicks, eggs and nests were killed or destroyed - less than initially feared.
However, the clean-up operation is being botched because of a turf war between the regional government of Andalucia in Seville and the national government in Madrid, the RSPB said yesterday in a joint 100-days-on progress report with the Spanish bird protection society SEO (the Sociedad Espanola de Ornitologia).
The two administrations have opposing plans for the Donana's future protection; the regional government wants to build a series of sediment traps and other defences upstream, while Madrid wants to seal off the National Park with a vast new dyke 35km (22 miles) long.
Either would cost tens of millions of pounds and EU funding would be available, but progress is stalled as the two tiers of government disagree.
In the meantime, 1,500 clean-up workers promised by the Andalucian administration for June have still not been recruited, and barely 20 per cent of the sludge has been removed, the societies said.
What remains is becoming more poisonous still owing to the chemical changes taking place, and the toxins are now being absorbed into marshland plants on which the birds feed, said Laurence Rose, head of the RSPB's European department.
The worst threat of all, however, was the autumn rains because flooding that invariably accompanies them could spread the uncollected sludge further across the reserve.
"It will be too late now to stop it," he said. "We are praying that it does not rain too hard, although normally the Donana needs the rain.
"The Spanish authorities have messed up the recovery operation by allowing petty politics to get in the way of cooperation."Reuse content