Deadly mustard gas has leaked from a First World War underwater “weapons cemetery” in the North Sea, close to the Belgian coast.

The munitions dump of 35,000 tons of unexploded bombs, shells and grenades, created after the Great War, has long been considered safe.

In March fears were raised that some TNT had escaped, leading to intense study of the submarine sandbank, where the century old weapons, which are mostly German, are buried.

It emerged on Monday that, as well as leaked explosives, traces of mustard gas, which was used as a chemical weapon on the Western Front for the first time in 1917 by the Germans, were found.

Chemical weapons were sealed in cement filled container and dumped in the ocean after the First World War by many European countries.

But after almost a century underwater, history is rising to the surface again.