One of Britain’s top divers has died while exploring a shipwreck off the Greek coast.

Carl Spencer, 37, had been leading a 17-man National Geographic expedition to explore and film the Britannic, sister ship of the Titanic, off the Greek island of Kea, when he suffered a severe attack of decompression sickness (DCS) .

Known colloquially as “the bends”, DCs occurs when divers surface too quickly.

The Britannic, larger than the Titanic and deemed equally “unsinkable”, sank in 57 minutes after hitting a mine in 1916 while serving as a First World War hospital ship. The wreck was discovered by Jacques Cousteau, the French underwater explorer, in 1975 but, at a depth of about 304ft (120 metres), it remains the “Mount Everest” of underwater challenges, pushing divers to the limits of endurance.

Mr Spencer, from Burton-on-Trent, Staffordshire, had led a previous expedition to the Britannic in 2003, as well as undertaking explorations of the Titanic and the Carpathia — the liner that went to the aid of passengers on the stricken vessel. He is listed as one of the world’s most accomplished divers on the website Diving with Legends and was also an active participant in pioneering decompression research.

Despite his experience, witnesses report that Mr Spencer started convulsing at depth while filming the bow of the Britannic yesterday and made a rapid emergency ascent, missing out essential decompression stops.

A military helicopter flew him to the Athens Naval Hospital, where was pronounced dead on Sunday evening.

The Greek Merchant Marine Ministry said today: "Everything was done to save his life. A fully equipped military Super Puma helicopter was rushed in to take him to the best possible hospital in Athens and even in flight, every possible method of artificial respiration was tried to resuscitate him.”

Mr Spencer had been taken the Athens Naval Hospital because there was a decompression chamber there, but never regained consciousness, the minstry said.

DCS occurs when gasses in the body undergo changes that cause bubbles to form in the blood or tissues during decompression. There are about 300 cases of DCS every year in Britain alone, but it is rarely fatal and can take hours or even days for sufferers to show any of the associated symptoms, such as rashes, joint pain, headaches and paralysis.

• British diver Carl Spencer dies exploring Titanic sister ship