This is Newsbeat's guide to what's in them. The letters are one of the most secretive parts of the UK's nuclear programme, called Trident. So we've got Matthew Seligmann, Professor of Naval ...

So how he'll save the £100bn cost of Trident to put towards the national ... with major concerns over paying off the UK debt (letter, 19 April). Also troubling are comments sometimes heard ...

One letter is safely locked on board each of the ... There are two scenarios where the Trident missiles will be fired. First is if the signal is given from the Prime Minister to launch a nuclear ...

‘Letters of last resort’ One of the first ... officers stationed in its four nuclear submarines, which make up its Trident deterrent. It will set out clear instructions for what the commanders ...

ALEXANDER McKay (Letters, September 3 ... warn that religious or political attacks may happen without our precious Trident. Mr McKay clearly has a short memory. The Twin Towers attack in 2001 ...

are accurate (that would be a first), the main argument against replacing the Trident nuclear submarines is not the annual or total cost, as suggested by Ian Lakin (Letters, September 1).

"The need for the letters arose as soon as we had our own submarine-based nuclear deterrent," says Prof Seligmann. The UK has four submarines capable of carrying Trident nuclear missiles.