British government archives have just been released revealing that the Soviet-led invasion of Czechoslovakia in 1968 prompted NATO to consider using nuclear weapons.

They document discord and dismay among some NATO military chiefs, some of whom felt the alliance lacked the conventional forces needed to respond to any Soviet advance into western Europe.

Other papers reveal that King Constantine of Greece was at the centre of an assassination scare in London during the crisis over the military coup in his country.

They say a French hit-man was hired to kill the King, possibly when he came to London for a meeting with the then Prime Minister, Harold Wilson, in June 1968.