The debate over the replacement of the Trident nuclear weapons is quickly becoming a hot political issue. The BBC reports that the government is considering delaying the spending decision until after the 2015 election, even though Liam Fox, the secretary of state for defence, has been intent on renewing the missile system. As the Liberal Democrats pack their bags for their annual conference, a poll released today shows 77% of their members oppose "like for like" replacement of Trident. The Lib Dems can put their money where their mouth is this weekend by voting for an emergency motion calling for the defence review to be extended to enable a full review of Trident replacement. And as the Labour leadership battle nears its end, only one of the Miliband brothers is calling for the same thing.

Fox has been unable to explain quite why he is so intent on renewing Trident, even to David Cameron and Nick Clegg. Over its lifetime, it will cost the nation a staggering £97bn. And none of us will, we dearly hope, ever actually use it, unlike many other public services. It is a slab of public spending which is both huge and expendable.

We might be able to find some clues to Fox's motivations in the remarkably candid statements from a former prime minister about why this useless and expensive piece of military hardware should be maintained in a time of austerity. Tony Blair, in his recent autobiography, explained why he proposed to renew Trident. He acknowledged that "the expense is huge, and the utility in a post-cold war world is less in terms of deterrence, and non-existent in terms of military use". Furthermore, it is "frankly inconceivable we would use our nuclear deterrent" without the United States using theirs.

Why, then, subsidise a piece of equipment that is largely useless, will never be used without the US, and at a time of financial hardship? In the end, Fox's reasons are probably the same as Blair's. "In the final analysis," writes the former prime minister, "I thought giving it up too big a downgrading of our status as a nation, and in an uncertain world, too big a risk for our defence."

Everyone is agreed that right now there is no conceivable nuclear threat to the UK from anywhere in the world. The last government stated this openly, declaring that "no state currently has both the intent and the capability to pose a direct nuclear threat to the United Kingdom or its vital interests". Why, then, keep the bomb?

"Well, the future is uncertain," is the usual reply. If this were a good enough reason we might expect the military top brass to enthusiastically fall in behind it. But many have done the opposite. General Sir Hugh Beach recently suggested that, "at a moment when the defence budget for equipment is heavily overdrawn and with other important areas of procurement apparently ring-fenced it is time to reflect on how thin the justification for Trident really is". Field Marshal Lord Carver, former chief of defence staff, was blunter. "Trident," he said. "What the bloody hell is it for?"

So it looks like the real answer to this question is Blair's confession that scrapping our nukes would be the "downgrading of our status as a nation". This, it seems, is the heart of the matter. Trident is useless and ruinously expensive – but it allows our politicians to strut around on the international stage. For that dubious privilege, if Fox gets his way, you and I will pay through the nose, cutting essential public services and making the world less safe in the process.