Imagine the scenario. Britain has been wiped out by a surprise nuclear attack.

The prime minister has been killed. Should Britain's nuclear submarine fleet launch its own missiles in retaliation?

It's a decision that will hopefully never have to be made. //

The UK has four submarines capable of carrying Trident nuclear missiles.

Since 1969, one of those subs has always been on patrol, gliding silently through the world's oceans. //

Every prime minister has to write four letters - one for each submarine. They are addressed to the Royal Navy commander on board. They are usually handwritten.

The letters are locked in a safe aboard the submarine and destroyed, unopened, every time a new prime minister comes into office.

It's not known exactly what they say.

"There are only so many options available," says Prof Seligmann

"Do nothing, launch a retaliatory strike, offer yourself to an ally like the USA or use your own judgement.

"Essentially, are you going to use the missiles or not?" //

"The submarine has to make a judgement that the UK has been hit by a nuclear strike," according to Prof Seligman.

"The commander does that by trying to make contact with the UK via Naval Command or listening out for radio signals."

It's thought one of the key tests is whether the Today programme on BBC Radio 4 is still broadcasting.

If all the checks fail, the commander will go to the safe, remove the letter and find out what the orders are.