Tom Peck: Why sharing with the French would present a number of problems

Both countries see their independent nuclear deterrent as an important component of national sovereignty

Among the first jobs a new prime minister undertakes is to write four separate letters to the commanders of Britain's four Trident missile-carrying submarines, each to be locked in a safe on their boats.

The letters are, in effect, a prime minister's last will and testament, instructions on what to do in the event that contact with the British Government cannot be established; that it no longer exists. Should one of the letters ever be read by a Trident skipper, he will find himself as the man with his finger on the button.

The four ballistic missile submarines operate from the Clyde Naval Base on the west coast of Scotland. At least one submarine is permanently on patrol, far away from the UK, allowing for retaliation in the event that the remaining fleet at Clyde is wiped out.

Each submarine carries up to 16 missiles and between 48 and a maximum of 160 nuclear warheads. The warheads vary in size and potency, to allow the retaliation to be in proportion to the attack. The "continuous at-sea deterrence" has been the centrepiece of Britain's strategic defence policy for more than 40 years. Its current operational cost is around £2bn.

Hardly any of the crew on board the patrolling submarines know their precise location as they move around pre-fixed "boxes" which are thousands of miles square. The warheads are no longer pointed at fixed targets. They can be aimed and fired within 15 minutes. Should they be fired, the targeting is controlled from within the Ministry of Defence – no one on board the submarine learns where the missiles have been directed.

Sharing the system with the French would present a number of problems. "Both countries see their independent nuclear deterrent as an important component of national sovereignty," said Malcolm Chalmers, a professorial fellow in British Security Policy at the Royal United Services Institute and an expert in international burden sharing.

"Then there's the special relationship. Britain and the US co-operate on several different components of the programme, including warheads and missiles, with a close exchange of information – much closer than either country has with France. If the arrangement were to trespass into areas in which we already co-operate with the US, we would have to have their permission.

"All these situations are extremely hypothetical, but what if the US and the UK, or even the US and France for that matter, were to engage bilaterally in a war with a nuclear power, and that power were to attack France or the UK while the other's submarine was on patrol. Would the other retaliate on their behalf?"

During the long years of the Cold War, the prime ministerial letter was believed to have given the commanders four choices: put yourself under US command, if it still exists; go to Australia, if it is still there; fire your nuclear missiles at the enemy; and, finally, use your own judgement.