Britain's secret weapon in the event of nuclear war can at last be revealed - the AA and its fleet of patrolmen on yellow motorcycle sidecars.

Britain's secret weapon in the event of nuclear war can at last be revealed - the AA and its fleet of patrolmen on yellow motorcycle sidecars.

At the height of the Cold War, our defence chiefs were relying on the Automobile Association and its network of helpful broken fan belt repairers to help them launch a nuclear attack.

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So crude was the Government's electronic communications network that the only practical alternative that Harold Macmillan, the Conservative prime minister between 1957 and 1963, could find in the event of all-out war was the AA's extensive radio network.

In the words of one security expert this week: "If the Russians had decided to launch a nuclear attack on Britain during a busy bank holiday weekend, it could have been disastrous."

The extraordinary revelation comes in a new book, Planning Armageddon, which details Whitehall's last-ditch plans to counter the Soviet menace of the early 1960s.

"Outside the London area, government communications were so unreliable that in the event of a surprise Soviet attack, Macmillan felt forced to rely on the AA's radio network to issue orders back to Whitehall," writes Len Scott, who wrote the book with Stephen Twigge. To contact Macmillan when he was moving about the country, civil servants depended on intercepting him with the aid of police or rail authorities. To provide permanent contact, they decided to put a radio in his official car permitting "messages to be relayed in plain language through the Automobile Association's network".

The precise role of the AA remains unclear. But Scott speculates that the AA, whose headquarters were opposite the Air Defence Operations Centre in Stanmore, had closer and more murky links with the military than either side has been willing to admit.

Planning Armageddon also explains how, before the Cuban missile crisis, the British authorities considered shooting guards at American nuclear weapon stores in England if they proved uncooperative. They also targeted nuclear weapons at China as well as the Soviet Union, and delegated control of Britain's nuclear weapons to the military.

By the early 1960s the Soviet Union's growing stock of intercontinental nuclear missiles was reducing the warning time for a nuclear attack from hours to minutes, and senior members of the RAF were worried what would happen in the event of a total breakdown of communications.

Based on recently released public documents, the book reveals that in August 1961 Macmillan gave the head of the RAF's bomber command permission to launch his bombers if the Russians attacked and he could not contact the PM.

In the event of a "bolt from the blue", the Commander-in-Chief was authorised to order his bombers airborne, to seek contact with the Prime Minister, his deputy or American authorities, and in the last resort to order nuclear retaliation.

A month later, during the crucial weekend of the Cuban missile crisis, the RAF Commander-in-Chief, Sir Kenneth Cross, had difficulty contacting his political masters and personally took the decision to put his Thor missiles on alert level three, with V-bombers standing at the end of the runway.

It may be that Macmillan did not fully understand the orders he had given, but it later emerged that this emergency action was strongly at variance with his wishes.

Planning Armageddon: Britain, the United States and Command of the Western Nuclear Forces 1945-64 is published by Harwood.