I don’t want you to think me hopelessly incompetent, but when I was defence secretary I managed to make a hash of launching Britain’s nuclear deterrent. I was on board one of the four enormous submarines that carry bunches of the Trident missile and the Royal Navy was demonstrating to me what would happen if the call came from No 10 to unleash mayhem against the enemy.

The secret codes arrived and were matched. The dual keys were inserted and the moment came when I was invited to press the button.

You may not have given much thought to the proper manner of initiating Armageddon. I gave the button a little stab as one might when pressing nine for an outside line. Be warned, reader, you need to press hard and long.

My effete attempt to obliterate Britain’s foes led the system to enter an abort sequence. Bells clanged, lights flashed and sirens whooped. I think it might have been quieter if I had fired the missile. It was not easy to disguise the fact that I had goofed. While the crew struggled to reboot the boat’s computers, I was led away for a less demanding briefing on the sonar array, this time strictly hands-off.

Perhaps it is wrong to be flippant about the dreadful destructive power that Britain keeps lurking under the waves 24 hours a day and 365 days a year. But the point is that nowadays nobody expects it to be used. That was the position when I visited the submarine nearly a decade ago.

The Soviet Union collapsed long ago. There is no threat from China. The new nuclear weapons states, from India to Israel, do not have the capability to hit us. Relations between Tony Blair and Jacques Chirac may be strained, but as yet we have no reason to fear a nuclear strike from la force de frappe.

So it seems rather surprising that according to some reports the government has decided to replace the Trident D5 missile and the submarines that carry it, at a cost of tens of billions of pounds.

Blair prides himself on being forward looking. His flexible mind impatiently discards the shibboleths of old think. Labour started its period in government with a review of defence policy to take account of the end of the cold war. But none of that new realism is allowed to affect the doctrine of Britain’s independent nuclear deterrent.

Blair is scarred by the experience of Michael Foot, whose policy of unilateral nuclear disarmament cost Labour dearly in the 1983 election. A Tory poster parodied his defence policy by showing an unarmed British soldier with his hands raised in surrender. Foot’s manifesto was described as the longest suicide note in history.

But in those days there was a Soviet Union and an arms race. Even so by 1986 President Reagan (not generally thought a patsy) sought agreement with Mikhail Gorbachev at the Reykjavik summit to abolish both countries’ nuclear weapons. He was restrained by Margaret Thatcher, who argued rightly that since the world cannot un-invent the technology, the United States must maintain a nuclear deterrent.

The arguments for Britain are quite different. Blair lacks the flexibility even to bring his thinking into line with where Reagan’s was nearly 20 years ago.

The case for Britain having an independent nuclear deterrent depended on the existence of the Soviet Union. That superpower had the capacity to launch warheads simultaneously to wipe out the cities and the land-based nuclear forces of the United States and western Europe.

Confronting such a well equipped enemy, we had to complicate his calculations. The Russians might not believe that America would risk nuclear war in defence of its European allies. So France and the UK acquired their own systems to reduce the uncertainty and increase the deterrence.