It is thought the underground trial at America's desert test range in Nevada was carried out on one of the new warheads for the Polaris submarine missiles.

The announcement has sparked a row between senior ministers and the left-wing Tribune group, which opposes Britain's - and Labour's - involvement in the arms race.

The trial had been arranged by the previous Conservative Government, but the Ministry of Defence said it was not ashamed of its nuclear arsenal and intended to continue with the programme.

'Grave danger'

Tribune member Frank Allaun said it was Labour policy to get rid of what he termed "suicide weapons".

And the MP suggested scrapping missiles would set a good example to aspiring nuclear powers.

"The danger at the moment is of the bomb spreading to more and more countries - that's a grave danger," he said.

Defence Secretary Roy Mason said he could understand why people in the Labour Party were opposed to the tests.

But he emphasized there was no party commitment to the abolition of nuclear weapons and said the government had not broken a manifesto pledge or international agreement by exploding the device.

"If we had purposefully taken the decision to abandon the test we would have been prematurely taking the decision to abandon our strategic deterrent - that's not on," he said.

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