Deadly mustard gas is leaking from a vast underwater munitions dump created close to Belgium's North Sea coastline after the First World War.

The so-called weapons cemetery – containing 35,000 tons of unexploded bombs, shells and grenades – has long been considered safe.

Mainly German weapons were buried there in cement-filled containers under a two mile sandbank close to the Belgian coastal town of Knokke.

But fears have been raised that the historic ammunition may create new problems after traces of both mustard gas and TNT were detected on the sandbank, known as Paardenmarkt.

The traces were found at two separate sites, Philippe De Backer, Belgium’s North Sea Minister, told parliament on Wednesday.

Indyplus gallery: First World War in pictures Show all 15 1 /15 Indyplus gallery: First World War in pictures Indyplus gallery: First World War in pictures Germany 1918: A scene from the German film 'West Front', about life in the trenches during the first World War Indyplus gallery: First World War in pictures Trafalgar Square, London 1914: In Trafalgar Square, London street urchins dressed as soldiers with paper hats and canes as guns stand to attention watched by a small crowd. Behind them is a notice declaring ' The Need for Fighting Men is Urgent' Indyplus gallery: First World War in pictures Marylebone Grammar School, London 1914: Two men conscripted to the British Army undergoing a medical check-up at Marylebone Grammar School, London Indyplus gallery: First World War in pictures Victoria station, London 1914: A soldier saying goodbye to a loved one in the rain at Victoria station, London, as he leaves for the front Indyplus gallery: First World War in pictures Victoria station, London 1914: Two soldiers on the concourse at Victoria station, London, about to leave for the front line. They are carrying parcels full of food and other provisions Indyplus gallery: First World War in pictures British Army 1914: A group of new recruits in training for service in the British Army during World War I Indyplus gallery: First World War in pictures Aisne, France 1914: A lone soldier with a bicycle stands amid the remains of a German motor convoy which lines a country lane after an attack by French field guns in the battle of the Aisne in France Topical Press Agency/Getty Images Indyplus gallery: First World War in pictures Aisne River, France 1914: German sharpshooters move to a position near the front line, during the fighting near the Aisne River Indyplus gallery: First World War in pictures German naval zeppelin 1914: The L2, a German naval zeppelin during World War I Indyplus gallery: First World War in pictures France 1914: French officers dining in style in a trench near the front line Indyplus gallery: First World War in pictures Fort Vaux, France 1916: A German rifleman beside the corpse of a French soldier in a trench at Fort Vaux, France Indyplus gallery: First World War in pictures London 1915: Soldiers arriving at a station in London to travel home for Christmas Indyplus gallery: First World War in pictures German Army 1915: A wounded German soldier Indyplus gallery: First World War in pictures British Army 1915: A wounded British soldier is stretchered back to camp past a carnage-strewn trench, during the World War I Indyplus gallery: First World War in pictures Brighton Pavilion 1915: Injured Indian soldiers of the British Army at the Brighton Pavilion, converted into a military hospital

Carl Decaluwé, the governor of West Flanders, called for the site, which is less than a mile from the coast, to be cleared immediately after the revelation.

“Better to do this now than when the leaks have got worse,” he told the Het Laatste Nieuws newspaper.

But Leopold Lippens, mayor of Knokke-Heist municipality, said decisions must not be made too hastily.

“There are dozens, if not hundreds, of ammunition cemeteries in the North Sea, especially along the French coast,” he said. “Why would there suddenly be a problem in Knokke?”

Although it is now more than a century since the Great War ended, it continues to haunt Belgium’s seas.

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