Theresa May to be briefed on protocol for firing nuclear weapons

When a prime minister crosses the threshold of 10 Downing Street for the first time, many formal procedures need to be run through. None is more secret than the protocol for firing Britain’s nuclear weapons.

Theresa May will be taken through these procedures by Sir Jeremy Heywood, cabinet secretary, and Gen Sir Nicholas Houghton, chief of the defence staff. They will inform her of the technical means – the codes – by which Trident can be launched.

The new prime minister needs to set out a plan for the retaliatory use of nuclear weapons should Britain be hit by a surprise strike. She must appoint two “nuclear deputies” to take charge if she is killed in an enemy offensive.

Letter of last resort

Jonathan Powell

, Tony Blair’s former chief of staff and a long- time diplomat, wrote about this in

The New Machiavelli

, his memoir. He said discussion about who to appoint “always leads to arguments with, to my surprise, ministers clamouring to be given the task”.

Ms May will then write "a letter of last resort" for use if she and both deputies are killed. It is copied four times and one is sent to each of the commanders of the four submarines carrying Trident at the Faslane nuclear base in Scotland.

In his book, The Prime Minister: The Office and its Holders Since 1945, Lord Peter Hennessy, describes the purpose of this letter.

"Should [the prime minister] be wiped out by a bolt from the blue, the Royal Navy commander of whichever Trident submarine is then on patrol in the north Atlantic, after some days scanning the airwaves for signs of life back home (the failure to pick up the BBC Today programme is regarded as the ultimate test), will . . . open the sealed instructions which that prime minister must make ready within a few days of taking office."

What form does this letter take? Lord Hennessy writes that the choice, in crude terms, boils down to "let them have it" or "sail to New Zealand if it's still there".

The letter of last resort is the greatest of all government secrets.

The entire UK nuclear deterrent depends on the principle that a sitting prime minister will inflict terrible vengeance on any state that tries to wipe out the UK in an attack.

Ms May will doubtless approach this task with the gravity it deserves.

– Copyright The Financial Times Limited 2016