Ernest Bevin, the Labour Foreign Secretary between 1945 and 1951, insisted that Britain must become a nuclear power because he wanted a “big bomb with a Union Jack on top of it”. The arguments for this country retaining its nuclear arsenal are rather more subtle and sophisticated yet ultimately boil down to something similar. Bevin was convinced that Britain could not exercise the influence that it should globally if only the United States and the Soviet Union had stocks of atomic weapons and, crucially, that the world would be a poorer place if that British influence could not be duly exercised. Gordon Brown has, in effect, reached a similar conclusion.

The Chancellor’s intervention, even if unexpected, is timely and welcome. He has been criticised by some in his own ranks who believe that it will be harder to dissuade the likes of North Korea and Iran from seeking nuclear status if Britain opts to renew its hardware rather than renounce it. This is, alas, a distinctly innocent outlook on international relations. It is not one that a serious politician in a position of power could articulate, and Mr Brown is certainly a serious politician.

The disarmament thesis falls down on three counts. The first is that there is not the slightest evidence that the rulers in Tehran or Pyongyang are in the remotest interested in whether or not Britain’s Trident submarines are replaced when they become obsolete a decade or so hence. The second is that the Nuclear Non-Proliferation Treaty explicitly recognises Britain’s place as a nuclear power while Iran pledged in that document that it would not obtain a nuclear stockpile. The third is that as a permanent member of the United Nations Security Council, Britain’s retention of nuclear weapons is a logical aspect of the “policeman” role this country plays in global affairs.

It is, therefore, as much for internationalist as nationalist reasons that a Labour government should not want to abandon its nuclear capacity. This is not merely a matter of “prestige” for its own sake but, as Mr Brown has calculated, a move necessary to ensure that Britain is able to exercise a positive influence in Africa and beyond because it sits at the global top table.

If the basic principle of Britain and nuclear weapons is relatively straightforward, the debate over which system might be adopted is more complicated. On all previous occasions when such a decision had to be contemplated, the presumed “enemy” was clear as were the circumstances in which that foe, the Soviet Union, might try to use the threat of Armageddon to blackmail others into submission. It is far harder today to assess what range of challen-ges a deterrent might help to repulse.

It may be that a comparatively “cheap and cheerful” renewal of the existing nuclear submarine fleet might be sufficient or, by 2015, it may be plain that a more extensive package is demanded. There is plenty of room for discussion about what is the right formula.

That decision does not have to be taken today. The process of thinking about it, however, should start soon. It can only be undertaken if the political will to proceed has been established. It is to Mr Brown’s credit that he had the courage to speak out on this question when it would have been much easier for him to hide behind his department brief and remain silent or obfuscate. Government is about making choices. Bevin was a master of that. Mr Brown has shown a capacity to emulate him.