BRITAIN'S first 'H-bombs', tested in 1957, were not H-bombs at all, but a bluff to convince the world that Britain was still a first-rate power, according to research to be published tomorrow.

At the time, the report's authors say, it probably even fooled the Americans for a while. It provoked an interest in the British nuclear programme that ended with the US giving Britain the means to manufacture true H-bombs in exchange for access to British ideas and designs for future weapons. According to the research, the secret of the bluff was confined to top nuclear scientists and inner government circles.

Furthermore, newly released US documents show that the British did not develop their later thermonuclear warheads - true H-bombs - entirely on their own, as the Government has asserted, but drew on US designs.

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A 1958 report, recently released from the US Department of Energy archives, serial number LXXXI-596, says: 'There are specific developments which the UK scientists have made which hold a great deal of interest for us and which might offer advantages in our weapons systems.'

The assertions appear in an article in the London Review of Books by Norman Dombey, professor of theoretical physics at the University of Sussex, and Eric Grove, formerly of the Royal Naval College, Greenwich.

There are two broad categories of nuclear weapons: those which use nuclear fission, called 'A-bombs', and, more powerful, those where most of the 'yield' - the bang - is provided by nuclear fusion of hydrogen and other elements, called 'H-bombs'. The H- bomb is not radioactive itself, but relies on an A-bomb to act as a trigger.

A-bombs have yields measured in kilotons, equivalent to thousands of tons of TNT; Hiroshima was destroyed by a 20 kiloton A-bomb. H-bombs are thousands of times more powerful, yielding explosions equivalent to millions of tons of TNT. However, by placing fusion components inside a fission bomb, it is possible to enhance its efficiency and thus its yield. Its main yield still comes from the fission, and so it is not a true H-bomb. These are 'boosted fission' weapons.

The US Department of Energy document refers to a 'high-yield fission bomb' and two 'boosted fission designs' which would correspond to three British tests in 1957. The first, on 15 May, 'Short Granite', would have been boosted fission; the second, 'Orange Herald', on 31 May, a large fission bomb; and the third, 'Purple Granite', in June, another boosted fission. At the time, the government maintained that all three tests involved H-bombs.

To this day, the Government refuses to supply the yields of the tests, describing them vaguely as 'in the megaton range'. One nuclear expert said yesterday: 'You can put your own construction on that fact alone.'

The second test was witnessed by journalists on board the frigate HMS Alert, who were told it was an H-bomb. The authors say it is likely this was a large, straightforward A-bomb, since it was uncertain how even a boosted fission device would perform.

In 1958, the Leader of the Opposition, Hugh Gaitskell, asked Harold Macmillan, the Prime Minister, why further H-bomb tests were necessary if we had it and had tested it already.

Macmillan said: 'The Right Honourable gentleman says we have the bomb. If he had accepted my invitation to discuss these matters . . . he would have spoken with a greater degree of responsibility.' This suggests that had Labour been prepared to talk to Macmillan, it would have been let in on the secret - that the tests in 1958 were Britain's first real H-bomb explosions.

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