In the search for spending cuts the Trident replacement is an attractive target. Politicians already dream of scrapping it and spending the greatly exaggerated “dividend”. Meanwhile, its benefits in terms of our security and position in the world are ignored as an economic inconvenience.

It is disappointing, but perhaps not surprising, that the Foreign and Commonwealth Office should let the Ministry of Defence do the heavy lifting in this debate: the MoD, after all, has the budget, the manpower and the operational expertise. Yet it is the Foreign Office that reaps so many of the benefits (on Britain's behalf) and so has the most to gain from replacement. Trident and its successor are as much about national power and Britain's position in the world as about military effect.

The five permanent members of the UN Security Council (the US, Russia, China, Britain and France) achieved their positions by being the victors of the Second World War. But they now retain those seats only thanks to their possession of credible nuclear deterrents. It's not about GDP, hospitals, improvements in child poverty or school league tables: abandon the deterrent and, sooner or later, Britain loses its seat.

Anyone who does not think the seat valuable should be open enough intellectually to assess the diplomatic value of a Security Council veto. Germany, Japan, India all want and - on most measures - deserve a permanent seat more than Britain. Cancel Trident's replacement and we join the second rank of European countries, on a par with Italy or Spain economically and militarily (to say nothing of abandoning our obligations under the Non-Proliferation Treaty to protect European countries that lack such weapons).

The debate on Trident, and in particular its costs, is contaminated by misleading figures that overstate the short-term costs - and hence the financial benefits of cancellation - while understating those of comparably large civil projects. Put simply, estimates of £30 billion and over (and the Liberal Democrats put the cost at £76 billion in 2006) combine the capital cost (of building submarines and infrastructure, and buying missiles) with the annual operating costs for a 25-30 year life.

On the same basis, a comparable civil project, the 953-bed Norfolk & Norwich University Hospital will cost not £229 million, as announced in 1998 (which ballooned to £1.16 billion - it is not just defence that has huge cost overruns) but £16 billion, including PFI charges, staff and equipment. Any debate should at least be on an apples-to-apples basis.

Replacing Trident can, and should, be done at the lowest cost for the highest national and military effect. Only a submarine-based system is likely to guarantee security, survivability and uncertainty (for our enemies) for decades to come. But the system will no longer have to overcome what were the world's only operational anti-ballistic missile defences around Moscow.

Since 1998 Vanguard-class submarines have carried no more than 48 nuclear warheads across their 16 missiles, only a quarter of their capability. Their replacements can be smaller (based on the new Astute hunter-killer submarines being built for the Royal Navy) and with fewer missiles (eight rather than 16 per boat) while achieving the same effect. Improved operational procedures, especially maintenance, overhaul and refuelling, also mean that we no longer need four submarines: three could guarantee the required “continuous-at-sea deterrence”.

On this basis the MoD's 2006 estimate that like-for-like replacement of the four Vanguard submarines would cost £15 billion-£20 billion could be reduced to as little as £10 billion, costing £40 billion over a 30-year life. Yes, that is two-and-a-half times the cost of a large hospital, but the fundamental job of the Government is to protect its people and country and no one, uniformed or civilian, can forecast accurately world events even a decade hence, let alone three.

The next government, of whatever hue, will find out all too quickly that there are no easy cuts left in defence. Any decision is likely to have implications not just for jobs but also for Britain's industrial and military capability. Cut the Trident replacement, and Britain's entire submarine manufacturing capability would be almost fatally undermined, making any future submarine programmes unaffordable.

This is not an industry like civil nuclear power, where we might be able to look for an overseas supplier to fill a gap caused by a short-sighted costs-driven decision to delay or abandon a programme. Similar choices face our defence industry in areas such as armoured vehicles, surface ships and helicopters.

The defence industry is an unlikely candidate for a sympathetic hearing from the British public about its need for lucrative contracts, but it represents much of what is left of our manufacturing capability, and creates, trains and sustains high-quality jobs. At least 15,000 jobs, directly and indirectly, could depend on the Trident replacement. Such a capability cannot just be funded in an economic upturn, then left to fend for itself when the money runs out.

A combination of poor programme management and a “conspiracy of optimism” continues to plague the MoD and its suppliers, and persistently high defence inflation (3.5 per cent a year above RPI) must be recognised and addressed at all levels. Far greater openness is surely the best way to achieve this. Debate on the Trident replacement should be the starting point.

Sash Tusa is a defence consultant and a serving officer in the Territorial Army

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