The British government was forced to admit it had sanctioned a nuclear weapons test in 1974 after a journalist dreamed one had taken place and wrote an article, newly released documents showed Friday.

Then prime minister Harold Wilson strived to keep secret the test in the Nevada desert in the United States that May, but his efforts were foiled by the unorthodox reporting methods of Daily Express journalist Chapman Pincher.

The newspaper's foreign editor, John Ellison, recounted Pincher's dream to senior Foreign Office official Crispin Tickell, who relayed the story in a memorandum to Wilson's Downing Street office.

"Mr Ellison said that, believe it or not, Mr Pincher's source was Mr Pincher himself," wrote Tickell, later Britain's ambassador to the United Nations.

"He had been on a fishing holiday in Scotland. One morning at breakfast he said that he had had a particularly vivid nightmare about a nuclear explosion which he was sure was British.

"His companions told him that he was obsessed and should get back to his fishing. He duly did so for three days, but on his return to London he rang up his friends in the Ministry of Defence with some such questions as: 'What was all this about a British nuclear test?

"Eventually he got an answer which was sufficiently equivocal to convince him that he was on the right track. He then wrote his article."

Despite the apparent improbability of the story, Tickell seemed to believe that the explanation was genuine -- he added: "Against extra-sensory perception and the best contacts in Whitehall, I feel that we are powerless.

"Mr Pincher remains the greatest."

Pincher reported that the test was about to take place when it had actually already happened, but his article provoke uproar in Parliament and Wilson had to go to the House of Commons to reveal what had taken place.

There was such secrecy surrounding the test that only three other members of the Cabinet had been told it had taken place.

The document was released by the National Archives in Kew, west London, under laws which allow official papers to be made public after 30 years.