Britain feared Soviets would unleash 20,000 Hiroshimas Soviets planned to hit Britain with 20,000 Hiroshimas

A GRIM reminder of the Cold War is revealed today in formerly top-secret Ministry of Defence papers that outline the expected devastation from a Soviet nuclear strike on Britain.

The MoD report from 1967, released under the 30-year rule at the Public Record Office at Kew, estimates that a Soviet thermonuclear attack against Britain would have unleashed a destructive force nearly 20,000 times greater than Hiroshima. It would have flattened much of urban Britain to little more than smouldering wreckage, with millions of casualties. It is the first time that such a candid government estimate of the scale of nuclear war on the British Isles has been released.

The list was compiled by the forces' Joint Intelligence Committee and approved by the chiefs of staff, Britain's most senior defence committee made up of the chiefs of the Navy, Army and Royal Air Force.

The document, titled Probable Nuclear Targets in the United Kingdom: Assumptions for Planning, outlines military estimates of how Britain's cities and defence installations would be razed by a Soviet nuclear air attack. The material is based on high-level intelligence and contains a table of likely targets, with the size of nuclear bomb expected to be used.

Military top brass thought the Soviets had identified 104 potential targets, including all Britain's main cities, air bases and naval bases. They estimated more than 360 nuclear weapons would be launched by the Soviets with a total yield of 389 megatons - equivalent to 389 million tons of TNT explosive. The bombs that were dropped on Hiroshima and Nagasaki at the end of the Second World War, by comparison, had yields of just 20,000 tons of TNT.

In an introduction to the report, the Secretary to the Defence Chiefs of Staff, Major General Gibbon, said that even that figure could be an under-estimate. "Note has also been taken of comment by the deputy Chief of Staff (Intelligence) that Russian strategic missiles are now assessed to have an operational yield of half to one megaton - although a maximum yield of about three megatons is possible," he said.

London was expected to take the heaviest punishment with a combination of eight one-megaton missiles and two 500 kiloton bombs - 450 times more powerful than Hiroshima

The MoD believed that the Soviets had selected central and regional government centres, including London and the military commands of the Royal Navy and RAF; some 20 cities from Glasgow to Nottingham, RAF bomber bases and USAF nuclear bomber bases; submarine and naval bases, including Rosyth and Portsmouth.

Defence experts believed the Russians were aiming two nuclear missiles at each target in case one failed, and most were also earmarked for raids by aircraft carrying atom bombs.

The minutes of a Ministry of Defence meeting that examined the report makes no reference to likely civilian casualties but, had only a quarter of these weapons reached Britain and detonated, at least a sixth of the entire population would have been killed instantly, with millions dying later from radiation sickness, burns and other injuries.