The spill happened in the islands of the Schleswig-Holstein national park on the German-Danish border, which forms part of the Waddensee, the enormous area of coastal shallows stretching as far as the Netherlands, which is northern Europe's most important wintering ground for seabirds.
Thousands of waders such as oystercatchers, dunlin and sanderlings, and sea ducks such as eiders and common scoters, have already been wiped out by heavy fuel oil leaking from a crippled Italian freighter, the Pallas.
The ships was carrying a cargo of timber from the Baltic to Casablanca when it caught fire off the Danish North Sea port of Esbjerg on the night of 25 October. But its owners refused assistance and towed it out beyond the Danish 12-mile limit in an attempt to salvage it themselves. They failed, and for nearly three weeks the stricken vessel drifted southwards into the small islands off the German coast, leaking fuel oil as it went.
Although the amount was not enormous by the standards of past oil spills such as the Sea Empress off Pembrokeshire in 1996 - about 50 tons - the spill happened in exactly the wrong place and the wrong time, according to Robert Oates, the RSPB's marine policy officer.
"It is very heavy oil, which does not disperse and it has split into several slicks and there are enormous concentrations of birds there," Mr Oates said.
"At least 20,000 are dead already and about another 130,000 are threatened. It is an ecological catastrophe and it far exceeds the damage to the Coto Donana in Spain by the pollution incident last spring. We are extremely concerned."
Sarah Scarth of the International Fund for Animal Welfare said: "This is the worst crisis with oiled sea birds I have ever witnessed. It is heartbreaking."
The Waddensee extends for 10,000 square kilometres and is the winter home to an estimated four million birds from all across Europe.
The Pallas has now been anchored to a Dutch oil spill control rig off the German island of Amrum and the oil leak is believed to be under control. But the seabird deaths are continuing from the oil already spilt, Mr Oates said.Reuse content