Dan Lewis: If we don't pay for Trident, Britain will count the cost

It may win him some brownie points with the Liberal Democrats, whose poll ratings are in freefall, and undermine Dr Fox, a stalwart and most able Conservative rival. But be in no doubt, should this decision stand, it will have decades long consequences for our defence and none of them favourable. For now our cash-strapped, ultra-lean Armed Forces, will have to be pared back even further and quite unable to do much at all, let alone deter, fight and win conflicts that will almost certainly not be of our choosing.

That's deeply worrying because today's global security backdrop is going to get worse. Nuclear proliferation is accelerating. Threats from further away by those countries numbering some four billion who don't share our democratic values will come closer to home. And Britain, a nation in decline, finds itself squeezed between Western Europe, which has long lost the will to commit troops in scale to defend its own interests, and Obama's America, which has proven itself to be all too flattering to its enemies and, at best, unsupportive of even its most stalwart allies.

And so to Trident which, with its large price tag, may look like a saving. Osborne insists he is "absolutely clear" that this must come out of the MoD budget. Perhaps then he might instruct his Treasury mandarins to share with us the cost-benefit calculation which shows that the UK would be better off succumbing to nuclear blackmail or a mushroom cloud over London?

The real cost of Trident's replacement is not so much the missile as the submarines that carry it. So this has led to some exploring whether it would be better to have nuclear-tipped cruise missiles or land or air-based launch equivalents. Unfortunately, none of these ideas stands up to rigorous debate. There are no nuclear cruise missiles available so they and the warheads will have to be developed at vast expense. In any case, they will have much shorter range, are subsonic and can be shot down with Second World War-era anti-aircraft guns. Moreover, in Britain's crowded, planning-obsessed islands, land-based missile silos are not a serious option. And the chances of the RAF being able to fly all the way round the globe, refuel, get overflight or basing rights to drop a nuclear bomb on a third party and maybe even come back and do it again is laughable.

For all these reasons and more, Trident – a weapon against which there is no defence – is still the best and only real deterrent.

The other high profile candidate for cuts is the couple of Queen Elizabeth class aircraft carriers. It's amazing that only 25 years after the Falklands War – directly caused by the failure to replace the large carriers in the 1960s, we're about to do this again. Costing 5.2bn, contracts have already been entered into for over 2bn so immediate savings here are not as great. The aircraft to go on them are a different story. The Royal Navy could easily switch to the much cheaper and highly capable US-supplied Super Hornet from the hyperexpensive joint strike fighters and save up to 7bn.

Above all though, it's the low-intensity warfare and disaster relief capabilities that aircraft carriers bring which are so vital. It's highly likely that during their prospective 40 year lifespans there will be many occasions where they have been able to evacuate civilians from warzones and lend support to peacekeeping and overseas aid operations. William Hague, the Foreign Secretary, and the Department for International Development should take an interest before it's too late. And the best thing any naval ship can do at sea is carry an aircraft which immediately opens up thousands of square miles to observation.

With long range fixed-wing aircraft rather than just helicopters, carriers can do this with ease. Developments in carrier-based unmanned aerial vehicles will take their strategic footprint even further.

Back in the General Election, I always thought Gordon Brown's best moment was during one of the televised leaders' debates. Nick Clegg was

waffling on about not renewing Trident at which point Brown said: "Get real, Nick" – to which he had no answer.

Britain's coalition Government needs to get real on defence too. Defence is the most important part of the Government's budget while only making up a mere five per cent of it. To cut projects and capabilities when they cannot be easily or quickly recovered would be a tragic error.

Detractors in the Treasury should also not forget that there's only one thing more expensive than fighting a war and that's losing it. Deterrence through strong defence is the cheapest option of them all. Future generations will long judge the decisions they make now and never forgive them for dropping our guard against possible annihilation.

Dan Lewis is chief executive of the Economic Policy Centre