It's two years since Parliament voted to start the process of replacing Britain's Trident nuclear weapons system. Despite the 'yes' vote, opposition was substantial within Parliament – including a massive backbench Labour rebellion. Outside Parliament, it was clear that 'No' was by far the majority sentiment. 72 per cent of those polled, just days before the vote in March 2007, opposed replacing Trident at that time.

That was the state of play in the last heady days before the credit crunch really kicked in. Even then, the cost of Trident replacement was a major factor in the public mind. The news that the total cost of a new system – when including life-time running costs - was likely to top £76 billion was a killer fact for many. Those faced with government spending cuts and the closure of local services were already counting the opportunity cost of a new generation of British weapons of mass destruction. The TUC Congress in 2006 voted overwhelmingly against Trident replacement, primarily because of the costs involved.

Since then it has become clear – thanks to a Public Accounts Committee investigation – that even the government's cost estimates at the time of the vote were only 'ball park figures'. It seems that cost and time over-runs are almost certain. In any case, the continuing costs of the existing system for the next twenty-odd years should be added to the tally too. In sum, we are talking in excess of £100 billion to fund Britain's nuclear habit.

Ever larger numbers of people are coming to the conclusion that this is not a price worth paying. And they are coming from an ever-wider political spectrum. Hard on the heels of former Labour minister Stephen Byers questioning the justifiability of such spending, leading Conservatives have now also shown that they are open to rethinking Trident and its replacement. A major debate is now reportedly under way in the shadow cabinet.

David Davis has written, in the Financial Times, that it is time for the Tories to look at their own 'sacred cows'. Davis questioned the justification for the 'wholesale upgrade' of a system designed to retaliate after a full-scale Soviet attack when today's likeliest adversary would be a 'much smaller, less sophisticated state. Should not the costs reflect this?'

David Cameron subsequently refused to rule out the possibility of a shift in Tory policy on nuclear weapons, promising a major strategic defence review once in office. He is being encouraged to rethink by senior backbench figures, most notably by James Arbuthnot, who is Chair of the House of Commons Defence Select Committee. Arbuthnot, who led scrutiny of Trident's replacement in the run up to the Parliamentary vote, described weapons system as of 'doubtful usefulness', but at that time opposed unilateral disarmament. Arbuthnot has since said that he thinks the debate on Britain's nuclear weapons should now be reopened.

Of course costs are not the only reasons to oppose the replacement of Trident. The key question has to be, does it actually add to our security and defence?

One strong perspective on this has recently come from retired senior military figures – two generals and a field-marshal. Trident, they observed in a letter to The Times, is militarily useless and should be scrapped. So if it is military useless, does it contribute to our security in other non-military ways? On the contrary. According to Kofi Annan, when Secretary General of the UN, while powerful countries say they need nuclear weapons for their security, other countries will come to the same conclusion. In other words, our continued insistence that the possession of nuclear weapons makes us safe, will encourage other states to proliferate. That would appear to be the view of North Korea: they think nuclear weapons will deter attack on their country and constitute a powerful international bargaining chip. That is an attitude that we reinforce and perpetuate at our peril.

In fact, when Gordon Brown became prime minister, Britain’s nuclear policy changed subtly but significantly for the better. It recognised the reality of the link between the failure of nuclear weapons states to disarm and the increased likelihood of nuclear weapons proliferation. Numerous statements by leading government figures reinforced this shift away from Blair’s erroneous view that we are legally entitled to keep nuclear weapons. Since then, Brown has been big on the idea of ‘leading’ global disarmament initiatives, although his thunder has been somewhat stolen by Obama, who is actually doing something about it.

Brown’s key weakness is that he talks the disarmament talk, but won’t deal with Trident. Sooner or later he will have to realise this is a contradictory position. When the majority of the British people – including Tory politicians and senior military figures - have come to the view that Trident should be reconsidered – or even axed – then Gordon Brown should not be holding back.