Britain has finally destroyed the last of its stockpiles of mustard and nerve-gas weapons left over from the Second World War and the Cold War years which followed, the Ministry of Defence will announce today.

The total elimination of the ageing chemical arsenal comes on the 10th anniversary of the UK signing a landmark international treaty aimed at eradicating all such weapons.

But small quantities of various kinds of lethal nerve agents and toxins will be retained at Porton Down, the government's secret research centre in Wiltshire, to allow scientists to develop protective clothing and other medical countermeasures for British forces.

A total of 3812 bombs and artillery shells filled with lethal gases have been destroyed at a cost of £10m. Most dated back to the 1939-45 era.

Although Britain officially gave up its chemical warfare capability in the late 1950s, it had earlier set up a secret manufacturing plant at RAF Portreath in Cornwall far from any population centres to avoid mass casualties in case of an accidental escape of nerve agent. In 1976, research was moved to Porton Down.

Britain first used chemical weapons in 1915 when it bombarded German trenches with chlorine gas shells in the battle of Loos on the Western Front.