A spillage of toxic waste in one of the most environmentally sensitive areas on Earth is threatening the wildlife of two continents. The affected area is the Donana National Park in Las Marismas, the Andalusian marshlands between Seville and the sea.

The Donana is more than an exceptional wildlife preserve, a key wintering location for the wildfowl of northern Europe and one of the last refuges of the Iberian lynx and the Iberian imperial eagle: for the park and its surroundings also form the main resting place for birds migrating between Europe and Africa.

Disaster struck in the early hours of April 25th when the retaining wall of a waste reservoir collapsed at a Swedish-Canadian mining plant north-west of Seville. Some 158,000 tonnes of waste containing heavy metals and other toxic material were sent oozing down the Guadiamar river towards the park.

But the event vanished from the headlines, largely because the lethal grey sludge was, for the most part, blocked before reaching the heart of the Donana. Only 3 per cent of the surface of the national park was covered. But gradually the effects of the disaster are seeping into every aspect of life in Andalucia.

Some effects are relatively small. For example, pilgrims travelling south from Seville in traditional covered wagons drawn by oxen or on horseback for last weekend's annual festival in honour of the Virgin Mary as Reina de las Marismas (Queen of the Marshes) were warned not to take their usual route across the Guadiamar to the town of El Rocio. Instead, they had to use the main road to avoid the thick layer of intensely toxic waste which still coats the banks of the river.

The official body co-ordinating the cleanup estimates that, at the present rate of slightly under 10,000 cubic metres a day, it can have the last waste taken off the surface by October 27th.

But Britain's Royal Society for the Protection of Birds estimates it could take as much as 25 years for the area to recover. A spokesman said: "We fear this will turn out to be the worst environmental disaster of its kind in Europe this century."

The weight of the toxic material which cascaded out of the Boliden Apirsa plant at Aznalcollar was almost four times as great as that released in the Exxon Valdez tanker disaster of 1989.

Some experts remain optimistic. Jose Antonio Valverde, the park's first director, believes "the chances of a wide-ranging disaster are minimal, if everything proceeds as it has done".

But such views are now in a minority. Spain's notoriously divided environmental pressure groups have joined in a declaration that the situation is much worse than claimed by the regional and national authorities because of the peculiar nature of the crisis, which is brimful of the potential for delayed effects.

"Heavy metals have a feature which is not noticeable at first," says Carlos Vallecillo, a biologist with the Asociacion para la Defensa de la Naturaleza (Adena). "They get into the body and act like hormones, causing problems of infertility, growth, sexual and neurological maturity, and even suppressing the immune system. They can also cause certain cancers."

The animals that died as the poisonous acid tide swept down the Guadiamar are likely to make up no more than a fraction of the eventual total. For the toxins have only started to pass up the food chain. The birds that come to the area to live off its abundant fish and shellfish are particularly at risk.

Park records show several species, such as the gull-billed tern and the black-necked grebe, go exclusively or primarily to the very area, just outside the park, where the toxic waste has banked up most thickly. An aerial count two years ago found 54 per cent of the cormorants and 46 per cent of the flamingos in the same area.

Many birds had just left the park to spend the spring and summer in northern Europe when the spill took place. But, starting probably with the grey heron, they will begin returning in August. And not even the authorities are expecting the mud to have been removed by then.

In the meantime, the metals in the mud - zinc, lead, copper and silver - will be seeping into the soil, creating a hidden peril for humans. According to Spain's Young Farmers' Association, some 1,400 acres of land which has not been covered in waste is irrigated by systems that draw water from wells feared to have been polluted.

The rate of seepage will depend largely on rainfall in the coming five months. It is usually dry from June to September, but the norm is for rain to fall again in October.

What would turn the disaster into a catastrophe would be if the heavy metals in the waste were to penetrate the aquifer under the park. Aquifer 27, as it is called, is the Donana's invisible secret. Up to 700ft deep, it covers some 2,000 square miles stretching from the Rio Tinto to the Rio Guadalquivir.

Initial tests suggest the toxins have not penetrated it. But nobody can be certain. As the head of Spain's Science Research Council, Cesar Nombela, remarked: "The fact that the first analyses indicate that the aquifer has not been polluted does not mean that one day it will not be."