John Hutton, the former def­ence secretary, gave warning on Wednesday that Gordon Brown’s offer to reduce the future Trident submarine fleet by one vessel could jeopardise the UK’s long-standing commitment to “continuous at-sea ­deterrence”.

As the prime minister prepares to outline his offer to reduce the fleet launching the UK nuclear deterrent from four submarines to three, Mr Hutton – who resigned as defence secretary earlier this year – warned that the system would not be effective if it failed to operate on a permanent round-the-clock basis.