It will take another five centuries to clear the Somme battlefields before the area is safe, bomb disposal experts warned on Thursday after collecting 25 tons of munitions this year alone.

Details of the potentially lethal "Iron Harvest" came as members of the Royal family including Princes Charles and Harry were due to arrive in northern France to commemorate the 100th anniversary of the bloody battle.

Michel Colling, head of the Amiens bomb disposal unit that handles the Somme, said: "Since the start of the year we've been called out 300 times to dispose of 25 tons of bombs.