The government can be assured of a comfortable majority in next week's vote on the acquisition of a new generation of submarines to maintain Britain's independent nuclear deterrent after 2022. Though perhaps 50 Labour back benchers could rebel on the issue, the Prime Minister can count on the support of the Conservatives. That, and Tony Blair's anxiety to leave his mark on history, are both poor reasons for rushing such an important decision. As today's report from the Commons Defence Select Committee demonstrates, more is at stake than simply whether or not the Trident system is upgraded. First, there is a lack of clarity about the weapons' use, not in some hypothetical doomsday scenario, but as part of a limited nuclear strike against an aggressive rogue state. Secondly, there is the question of whether the government is being disingenuous in its promise to cut Britain's nuclear stockpile by 20%.

Next week's debate will focus on the four Trident submarines, each carrying enough nuclear hardware to make Armageddon look like a minor skirmish. It is almost inconceivable that those warheads would ever be fired in anger. Far more relevant to the current global situation are the smaller tactical nuclear warheads carried by the UK's battlefleet. Of course, a degree of ambiguity regarding the use of such weapons is necessary to keep one's enemies guessing. Otherwise, their power to deter nuclear blackmail is reduced. But if there has been a change in UK policy about the circumstances under which what are basically battlefield weapons could be used, parliament and the public are entitled to know about it. The particular fear is that they could be used in a pre-emptive strike against a threat from a country such as Iran. Perhaps we should say "perceived threat", as in the case of Iraq the nature and imminence of the threat turned out to have been hugely exaggerated by our government.

The Prime Minister is likely to argue next week that reducing the number of operationally available warheads from fewer than 200 to fewer than 160 (and cutting the number of submarines from four to three) corresponds to cutting our nuclear stockpile by 20%. While technically true, this is misleading because, as at present, at any one time only one nuclear submarine will be at sea, carrying exactly the same number of warheads, 48. On what moral basis can Britain deter other countries from acquiring their own nuclear arsenals if we are doing so little to limit ours?

The cost predicted, £15bn-£20bn, is also almost certainly a major underestimate, especially as it fails to include upkeep and depreciation costs that could add £60bn to the bill over 30 years. Tony Blair is right to say nobody can predict who our enemies will be in 20 years, but for the future we can see we are going to need an army that puts soldiers on the ground and helicopters in the air to resupply them and we do not have enough of them. As we have said before, nuclear bombs are of little use against men in caves.