THE cost of replacing the Trident submarine fleet will be the most expensive defence project in British military history.

As seems to be the way with all major defence contracts, the price has been steadily revised upwards from £20bn in 2006 to £31bn, plus a £10bn “contingency fund”, in 2016.

Further delays would have added even more noughts to the final bill, so last night’s decision to go ahead at least brings to an end the protracted wrangling over the project’s future.

Theresa May claims Trident keeps us safe -– but does it really? In a world of asymmetric warfare, where two belligerents have vastly different military power, nuclear weapons are all but useless.

France’s nuclear arsenal is ineffective against the kind of terrorism used by Isis to wage war on the West.

Of course, no one knows what threats we will face during the 30-year operational lifespan of Trident’s replacement, although we hope and pray that none of them requires a nuclear response.

But in rushing to embrace a bespoke submarine fleet, politicians ignored cheaper alternatives, such as nuclear cruise missiles which can be launched from existing submarines and spread across more boats making them harder to attack.

The Royal Navy, too, may come to rue the decision to replace Trident because such a costly project will inevitably have a detrimental impact on the surface fleet. With two new aircraft carriers about to enter service, the Navy cannot afford further reductions in the number of escorts they require.

The row over Britain’s independent nuclear deterrent will rumble on for as long as we retain the ability to deploy the ultimate weapon of mass destruction.

The only thing everyone agrees on is the hope that it will never be used.