Details of the bomb, described only as a "nuclear device", are sketchy. The term "device" indicates that it was an experimental explosive rather than a fully developed weapon.

It was almost certainly part of the thermo-nuclear weapons programme which was started in December 1954 to develop the megaton hydrogen bomb, which is as powerful as one million tons of TNT.

The test was carried out at high altitude over the largely uninhabited Christmas Island to minimise nuclear fall-out.

This is the most important range of tests carried out by Britain, developed with limited resources and in a remarkably short space of time.

Scientists have taken two years to develop the tests compared with their American counterparts who took seven years before exploding their first device.

The bomb was dropped by a four-engined jet, Valiant of No 49 Squadron RAF Bomber Command, normally based at RAF Wittering, Northants.

The Minister of Supply, Aubrey Jones, was informed of the Pacific nuclear trials by Air Vice-Marshal WE Oulton, commander of the task force and WRJ Cook, scientific director of the program.

Scientists are evaluating the results of the testing and will make a further statement in the next few days.

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