On Monday, the Prime Minister launched the debate on the replacement for Trident. There seemed to be an assumption abroad that ,while Labour is divided on this issue, the Conservatives are unanimous in supporting the next generation of the Trident deterrent. There were already signs on Monday that this would be an unwise misapprehension.

There is in the Conservative Party a significant and growing group - of whom I am one - who have a healthy scepticism about Trident in today's and, more importantly, tomorrow's world.

The central question is whether in this post-Cold War and asymmetric world we still need an independent nuclear deterrent? Trident was designed for that Cold War where the enemy was known and the threat quantifiable. The static nature of the Cold War provided the ideal theatre for the doctrine of containment and deterrence;. Each side could effectively be contained by the credible deterrent threat of Mutually Assured Destruction.

Today the enemy is often indistinctly known and the danger unquantifiable. The threat to use nuclear weapons in these circumstances is not only illogical but incredible. Paradoxically the need for genuinely independent alternative deterrence is if anything greater. As the challenge become less comprehensive, so the need for flexible deterrence increases. Trident does not best fit that bill. We should be exploring other options which need not be, and arguably should not be, nuclear; they could even be "sub-strategic", less destructive and therefore more credible.

Anyway, the independence of our nuclear deterrent has long been a myth. It may, as the Prime Minister claims, be operationally independent. The idea that we would have even threatened to use it without the agreement of the US is laughable. What it did was to sustain our permanent membership of the UN Security Council, a qualification distinctly unhelpful in a world which seeks non-proliferation.

The Government needs to prove Trident is still the best deterrent. Its great strength was its ability to hide beneath the oceans, oceans which today are increasingly transparent from space. There soon will no longer be anywhere for submarines effectively to hide; and such vulnerability substantially reduces Trident's value as a deterrent. Any new version of Trident must be able to answer that charge. We got no answer to this on Monday.

We also have to ask whether a new Trident, or any other nuclear replacement, would be sufficiently flexible and adaptable in tomorrow's predictably asymmetric circumstances to be an effective deterrent? A nuclear deterrent is, by definition, a weapon of mass destruction whose comprehensive threat worked well in the Cold War. It would simply not be a credible deterrent in a vastly more fluid future, where no one would believe - with justification - that we would ever use it. The prospect of even threatening to use such a destructive nuclear weapon against a rogue state or a terrorist outpost is fanciful. We should now urgently be exploring our options, either in existence or in the course of development, which could do the job as well or better and possibly at less cost, and which, if used, would not poison the world for generations.

To be an effective and credible deterrent, such systems need to meet certain harsh criteria. The delivery platform must as far as possible be invulnerable to pre-emptive action. It must be highly mobile, highly maneuverable and highly accurate. The ordnance must be capable of mass destruction, medium destruction or pin-point accurate destruction.

There are projects under development, including those involving the kinetic energy of hypersonic mass, which should be part of the debate. Apart from anything else, if they worked they would cost less and would be genuinely independent of the United States.

Given the very long lead time for Trident's replacement to be deployed, there is plenty of time to develop such options. At the very least, their potential viability should be fully explored before any final decision on the replacement of Trident is taken. All the options should be fully and openly debated. The Government seems, somewhat patronisingly, to think that widening the debate to consider other options or variations would cause confusion. Refusing to do so creates mistrust and suspicion. On this crucial issue we are owed a full and open debate.

Until we have one, my serious doubts about "Son of Trident" will remain. Nuclear deterrence was for the 20th century. I do not believe that it is for the 21st.

The writer is a former shadow defence secretary