"The effect of this (nuclear assault) on dense populations would remain beyond the imagination until it happened," it said. "The entire nation would be in the front line. Life and property would be obliterated by blast and fire on a vast scale."

It said the explosive force of an attack using 10 large hydrogen bombs would be 45 times greater than the total tonnage of bombs dropped by Allies on occupied Europe in World War Two.

One hydrogen bomb alone falling on a built up area would cause 100,000 fires.

The report said the Soviet Union's aims would be simple:

"In the event of general war, the Soviet aim would be to put the United Kingdom completely out of it," it stated.

Papers from the Home Defence Committee on which Strath sat estimated that 12 million Britons would die if 10 hydrogen bombs were dropped on major city centres with no prior warning.

It went on to describe how the blast and heat generated from the bombs would account for nine of the 12 million deaths, nearly one third of the population of Britain at the time. The remaining three million losses would be from radiation.

MILITARY RULE?

A map showed areas deemed most at risk of a nuclear attack - London, Birmingham, Liverpool, Manchester and Glasgow.

Provisional evacuation plans envisaged trying to move around 16 million people to more secure locations.

The once sensitive report, released early by the Public Record Office because it is no longer seen a threat to national security, described how the military would take over the running of much of the country in the aftermath of a nuclear strike.

Where civil control collapsed, "the local military commander would have to be prepared to take over from the civil authority responsibility for the maintenance of law and order."

Generals and officials expected to have up to six months' warning of an increased risk of global war, as the "international situation" started to deteriorate, and seven days' warning that an attack was about to take place.

If it did come, the economic and social devastation would be such that Britain could not be expected to play any further significant part in an ensuing world war.

One paper stated: "...if in fact the scale of devastation in the United Kingdom was that we now visualised, such recovery as the country might make would be insufficient to enable us to resume a major role in the war."

But amid the grim catalogue of annihilation was a glimmer of hope.