The Baltic Sea faces contamination by thousands of tons of corroding chemical weapons dumped on the ocean bed after the Second World War.

Research carried out by marine scientists has found that thousands of shells, many containing mustard gas, have now started to leak and pollute the surrounding seabed.

Historians estimate that in 1947 Britain and the Soviet Union dumped up to 65,000 tons of German chemical weapons and chemical weapons agents into the Baltic under an international agreement.

There have long been fears that the metal cases of the shells, missiles and drums containing the highly-toxic chemicals would corrode, and now scientists studying the Gotland Deep, the area of the Baltic where many of the munitions were dumped, have said those fears appear to have been realised.

“Our research has shown that in the Gotland Deep there are about 8,000 shells and missiles that could pollute the environment,” said Dr Jacek Beldowski, for the Polish Institute of Oceanography.

“We have now confirmed that these objects are contaminating the seabed. Until now we could only speculate this would happen. As part of the project we also studied fish swimming in the area of the dumping site,” he added. “We found that they have more illness than fish in other areas of the Baltic and genetic defects.”

The cold waters of the Baltic turn mustard gas into a dense solid but it can escape and remains highly toxic and so poses a serious long-term threat to the marine environment.

There have been sporadic incidences since the war of Baltic fisherman suffering burns from lumps of solidified mustard gas caught in their nets, and once a mustard-gas seeping from a canister washed up on the shores of the Polish seaside town of Darlowo burnt about 100 people and left four victims blind. However, Dr Beldowski said a greater danger to human health stems from the possibility of eating contaminated fish.

Deep-water fishing is prohibited around the dumping sites and general fishing discouraged, but complicating the issue is the fact that it is unclear just where all the chemical weapons lie.

“Unfortunately the Russians, once they found out what they had onboard, often threw them [the weapons] overboard as soon as land was out of sight,” said Dr Beldowski. This has raised the prospect of hundreds or even thousands of undetected weapons leaking toxic waste into the shallow waters of the Baltic and into the fishing fields.

However, Captain Jacek Fabisiak, from Poland’s Naval Academy, stressed there was little risk to humans.

“We are not threatened with an ecological disaster on the scale of Chernobyl,” he told Polish Radio. “But, of course, fishermen, and seabed researchers have to be very careful. Sooner or later this problem will have to resolved.”

Removing any leaking weapons from the seabed could prove difficult as scientists have warned that the operation could spread contaminated silt, and that the rusting cases and drums could break up during their journey to the surface.

The Polish research heaps more environmental woe on the Baltic. Shallow, almost enclosed and fed by numerous, and sometimes very polluted, rivers the sea is particularly vulnerable to pollution.

There have also been accusations that during the early 1990s Russia dumped chemical and nuclear weapons into the sea following the break-up of the Soviet Union, although this has been denied by former officers of the Russian Baltic Fleet.