"It was terrifying. The pain was unbearable and my hands blistered all over," said Danish fisherman Walther Holm Thorsen, who was 15 when he threw a cracked grey canister back into the Baltic Sea after it was snared in the net of his trawler.

One of the first postwar victims of the Nazis in the 1969 accident, he said the pain came in the middle of the night, hours after he and another crew member had rinsed the oily substance off the fish. They had no idea it was mustard gas.

Thorsen spent three months in hospital, and his hands are badly scarred despite skin grafts. "Working as a fisherman now is hard - my hands often feel like they're freezing," he said.

He said that trawler crew are now more aware of the dangers from chemical arms and have decontamination gear aboard. "But increasing rust will be a problem in future," he added.

In some parts of Europe, no one even knows where tens of thousands of tonnes of munitions are.

Ole-Kristian Bjerkemo of the Norwegian coastguard said he hoped a new seismic survey would be carried out this year to locate ships loaded with Nazi stocks of mustard gas and the nerve agent tabun which were scuttled off Norway in 1945.

DUMP SITES A MYSTERY

Norway knows the exact locations of just 15 of a probable 36 ships in waters about 600 metres (1,970 feet) deep off the southern town of Arendal, one of the main postwar chemical dumps with 168,000 tonnes of Nazi ammunition.

"We want to know where they are," Bjerkemo said. A robot camera sent down in 2002 found a trawler net caught on one wreck. Sulphur mustard and traces of arsenic compounds were found in the seabed but no chemicals in the sea water.

European governments reckon the stocks are safest where they are, slowly seeping poisons that may break down in contact with sea water or become diluted over decades.

The environmental group Greenpeace says they should be recovered. Apart from the threat to people working at sea, a sudden release of nerve gas could kill fish stocks. Other poisons might sink into the sediment and damage the food chain.

"Recovery of dumped munition is a costly and high-risk operation which could result in the release of large amounts of toxic compounds," the OSPAR commission of 15 nations protecting the north-east Atlantic said in a study.

"This problem is not going to go away," countered Paul Johnston, principal scientist at Greenpeace research laboratories. "As corrosion sets in the likelihood of releases increases.

And he said that, unlike Iraq, "we know the weapons are there." U.S.-led forces have failed to find alleged weapons of mass destruction that were a main justification by President George W. Bush for the war to overthrow Saddam Hussein.

Most dumps around Europe are from Nazi Germany but other countries from Britain to the United States have disposed of munitions at sea since World War One.

Led by Ireland, OSPAR governments are working on a common set of guidelines for fishermen on the frontlines, likely to be ready in June.

DON'T RUB STINGING EYES

The so-called Helsinki Commission, grouping states around the Baltic Sea, already gives tips to fishermen including:

- cut the nets if you suspect mustard gas, which smells like cress, horseradish or mustard.

- don't rub your eyes if they sting and you suspect mustard gas because you can go blind if you unwittingly already have it on your fingers. Instead, wash eyes with water from a hose for 15 minutes.

- fishing boats should have one "gas box" per three crew members that should include decontamination liquids and sprays and syringes with injections to counter nerve agents.