TONY BLAIR, like all his predecessors as Prime Minister, wants to pre-empt a full discussion on the future of Britain’s nuclear deterrent. He has said that a decision will be taken in this Parliament, later this year. But the promised “free and open public debate” will probably occur after, rather than before, decisions are made.

That is nothing to do with the largely irrelevant issue — raised by David Cameron and the Labour Left — of whether the Commons has a vote on whether Trident is replaced. Jack Straw, the Commons Leader, has already hinted at such a vote in saying that the debate on the White Paper will be “in a form that shows proper respect for the House”.

But whatever the Government does, it is not beyond the ingenuity of the opposition parties to initiate such a debate and force a vote, which would, incidentally, highlight Labour divisions.

The real question is different. Will the options be fully laid out and debated before the Government announces its decision? The omens are not good. In a report today the Defence Select Committee points to the refusal of the Ministry of Defence to participate in its inquiry beyond a general background paper.

In one sense we know the Government’s intentions. The Labour manifesto, and Mr Blair as well as Gordon Brown, have talked about maintaining the independent nuclear deterrent. Any government would stick to the formula set out so vividly 60 years ago by Ernest Bevin during the initial decision to develop nuclear weapons: “We’ve got to have the bloody Union Jack on top of it.” Or, more crudely, Britain is unlikely to leave France as the only European nuclear power. But in the post-Cold War world that case has to be made again. What threats can nuclear weapons any longer deter?

Morever, as the committee points out, this is only one of a series of decisions that have to be taken. First, does Britain continue the current policy of dependence on the United States for servicing of the missile and acquisition of parts for the warhead, while maintaining operational independence in the last resort? Second, how long can the current Trident system be kept going?

A new warhead will not be required until well into the 2020s, while the D5 missile will be in service in the US until 2042. The key issue is the platform, namely the submarine. What is called a “service-life extension” would add an extra five years to the current Trident and allow Britain to postpone a decision on replacement until about 2010, but no longer, since producing a new system takes about 14 years. This would not be cheap and the Government would have to continue to invest now in the infrastructure and skills base.

The key dates are: 2007-10, on whether to extend the current Trident system; 2010-14, on options for future platforms if the life of the Trident submarines is extended and we decide to retain a strategic nuclear deterrent; 2014, for a binding decision on details of a new system and spending; and between 2020 and 2025, for a new system to be in service, depending on extension of Trident and whether a submarine is always kept at sea.

This timetable is tight, but allows time for a full debate about all the options before the Government decides.

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