London: Downing Street was shocked to learn in 1962 that its American allies had been poised to start World War III over the Cuban missile crisis without consulting Britain, newspapers reveal.

So important did the prime minister, Harold Macmillan, consider a report from his intelligence chief in the days after President John F Kennedy faced down a Soviet nuclear threat in his backyard that he immediately sent a copy to Queen Elizabeth.

The warning from Major General Sir Kenneth Strong, the director of the Joint Intelligence Bureau, on November 19 struck particularly hard because it was delivered before the real end of the crisis. Unknown to No 10, three days earlier the US navy had conducted an exercise in North Carolina that was a dry run for an invasion of Cuba because its communist leader, Fidel Castro, was showing signs of reneging on the deal to remove the missiles and Ilyushin IL28 bombers that had pulled the world back from the brink.

Sir Kenneth, who had been General Dwight D Eisenhower's intelligence chief at the time of D-Day, had received detailed briefings on how the Kennedy administration was handling the crisis, probably from his friend John Alex McCone, the director of the CIA.

The papers, released at the National Archives in Kew, show that the retired general told Macmillan and Earl Home, the foreign secretary, just how serious the situation had become in the full heat of the crisis. A "note for the record" of the meeting reports his saying that President Kennedy "had concluded" before deciding to blockade shipping to Cuba that it would mean a likely Soviet attack on West Berlin.

"The Americans were prepared to go it alone, either without consulting their allies or irrespective of what their allies said, had the Russians reacted against any action in Cuba by moving against Berlin," the note reads. "Gen Strong thought the American government were prepared for their action in Cuba to escalate into the nuclear.

It seemed to him that the US administration was over-confident that they had pinpointed the position of all the main sites of inter-continental ballistic missiles in the Soviet Union and they hoped they would be able to take these out with a pre-emptive attack by their bombers."

Macmillan and Lord Home were horrified that their allies, who had appeared to consult them from the early stages of the crisis, were only paying lip-service to the idea.

The crisis had begun in late September 1962 when the CIA learned that Soviet technicians were installing nuclear missiles on the northern coast of Cuba. U2 spy plane flights over the island in October confirmed the presence of the missiles and on the 22nd of that month, as behind-the-scenes threats and demands were exchanged by Washington and Moscow, President Kennedy went on television to reveal that the world was on the brink of war.

Nobody doubted that it would be a nuclear conflict. After six days of stand-off as US navy vessels blockaded Cuba, the Soviet leader, Nikita Khruschev, agreed to dismantle and remove the missiles and bombers in exchange for a secret American disarmament in Turkey.

The blockade continued, with the constant risk of renewed conflict, until November 20, the day after Macmillan's meeting with Gen Strong. Len Scott, professor of international politics at the University of Wales, Aberystwyth and the author of Macmillan, Kennedy and the Cuban Missile Crisis, said on Sunday: "The papers show how worried the British were about how the Americans reacted to the crisis and it is certainly the first time we knew this."