For anyone old enough to remember the Cold War in general – and the 1963 Cuban Missile Crisis in particular – the phrase ‘mutually assured destruction’ is likely to be indelibly etched on their psyche.

The concept is chillingly simple: if two opposing sides have sufficient nuclear weapons to annihilate the other, full-scale nuclear war will be avoided, as a pre-emptive strike by either would guarantee its subsequent destruction. It also means, of course, that neither nation has any incentive to disarm.

Until recently, talk of all-out nuclear war had, thankfully, all but disappeared from public discourse – yet, like the weapons that enforce it, the concept of mutually assured destruction remains in place.

For the past half-century, somewhere in the world, a British Vanguard-class submarine carrying up to 16 missiles, each armed with up to eight nuclear warheads, has been on patrol below the ocean’s surface. The logic goes that the vessel acts as a deterrent to a nuclear attack, because even if the nation’s conventional defences were destroyed, a submarine could still launch a catastrophic retaliatory strike on the aggressor.

“Our nuclear deterrent guarantees the defence of the UK, and it has done so successfully for more than half of my life and yours,” wrote Prime Minister Margaret Thatcher in an open letter in 1986.

“To be effective, our deterrent has to be adequate to overcome the defences ranged against it.

“The Soviet Union has invested hugely in defence against ballistic missiles. It is now upgrading these defences further. If we are to retain the ability to penetrate them, we have to modernise our weapons systems. That was why we chose Trident. It will ensure that our deterrent remains effective well into the next century.”

Vanguard: renewing Britain’s nuclear deterrent

More than three decades later, the tense stand-off between the US and North Korea and concern about Iran’s nuclear programme have reignited the nuclear defence debate. In March, Prime Minister Theresa May reaffirmed the government’s commitment to maintaining the UK’s nuclear deterrent.

“The Chancellor of the Exchequer and I agreed the Ministry of Defence (MoD) will have access to £600m this coming financial year for the MoD’s Dreadnought submarine programme,” she said.

“Today’s announcement will ensure the work to rebuild the UK’s new world-class submarines remains on schedule and another sign of the deep commitment this government has to keeping our country safe.”

A final decision on the future of the Trident submarine fleet could not have waited much longer.

The current generation of four 150m-long Vanguard-class subs – Vanguard, Victorious, Vigilant and Vengeance – were introduced in 1994 and are due to be taken out of operation sometime in the late 2020s. The cost of replacing them with four new Dreadnought-class vessels is estimated to be £31bn.

Only one vessel is on patrol at any one time and work on a replacement could take up to 17 years.

Like its predecessors, the Dreadnought-class submarines will be equipped with multiple UGM-133A Trident II or Trident D5 submarine-launched ballistic missiles (SLBMs). Built by Lockheed Martin, they have a range of up to 7,500 miles. The subs will be the largest ever built for the Royal Navy, measuring 152.9m and with a displacement of 17,200 tonnes. They will cater for 130 crew, and manufacture their own oxygen and fresh water.

The £50.9bn price tag: can the MoD afford Trident?

However, funding the vast project is proving difficult. A recent report by the UK National Audit Office (NAO) reveals a significant shortfall in funding to renew and maintain the Trident nuclear deterrent.

Entitled ‘Equipment Plan 2017-2027’, the report puts the total projected cost of the Trident nuclear submarine project – designing, producing and maintaining the fleet of nuclear subs that carry warheads – at £50.9bn, £2.9bn more than the available Ministry of Defence (MoD) budget.

Around a quarter of the UK’s defence equipment budget to 2028 is earmarked for nuclear projects; this financial year, the MoD is expected to spend £1.8bn on procuring and supporting submarines.

The MoD has already taken £600m out of a £10bn contingency fund to pay for the Dreadnought programme and may be forced to go cap in hand to the Treasury again to plug the £2.9bn spending gap, the NAO warned.

“The coming years are crucial,” said Sir Amyas Morse, comptroller and auditor general of the NAO. “As the department invests heavily in the Dreadnought-class submarines and more widely across the enterprise, it needs to ensure that the new structures, processes and workforce operate effectively together to manage the £2.9bn affordability gap across the enterprise.”

The chair of the Commons Public Accounts Committee, Meg Hillier, commented: “The budget pressures on the MoD’s nuclear programme are significant. The department will need to make some critical decisions to get the programme on track financially.”

Are there viable alternatives to Trident?

The UK defence industry is already feeling the pinch. To fund the Vanguard replacement programme, the MoD has already had to find £3bn in efficiency savings over the next decade. Then there is the employment and brain drain dimension. Estimates suggest that up to 15,000 jobs – as well as significant expertise – may be lost if new submarines were not commissioned.

However, the NAO also flagged up skill shortages that need plugging across seven military sectors, with a need for 377 more skilled staff. The Independent reports that the MoD has developed new ways of working with key contractors such BAE Systems and Rolls-Royce on the Dreadnought programme in an effort to rectify past “poor performance”.

So, if the money can’t be found, what are the alternatives to Britain’s Trident nuclear deterrent in its current form?

The idea of using cruise missiles based on different subs with a far shorter range of around 1,000 miles was dismissed as even more costly, in terms of research and development, than renewing Trident. Cruise missiles, it was also pointed out, are also slower and more vulnerable to being shot down.

Other suggestions include a strategically located, land-based missile delivery system or launching missiles from a long-range aircraft. However, a 2013 options review again pointed to their shorter range and vulnerability, concluding that the idea needed “much more work”.

The Royal Navy has been committed to continuous at sea deterrence since April 1969. With military budgets under severe pressure, the UK Government may have to dig deeper into its pockets if the  Trident nuclear deterrent is to be upgraded and maintained for a further 50 years.