What does the new British Prime Minister write to his nuclear submarine captains upon taking office?

The letters of last resort are four identically-worded handwritten letters from the Prime Minister of the United Kingdom to the commanding officers of the four British ballistic missile submarines. They contain orders on what action to take in the event that an enemy nuclear strike has destroyed the British government, and has killed or otherwise incapacitated both the prime minister and their designated "second person", typically a high-ranking member of the Cabinet, such as the Deputy Prime Minister or the First Secretary of State, to whom the prime minister has designated the responsibility of choosing how to act, in the event that they die in office. In the event that the orders are carried out, the action taken could be the last official act of the Government of the United Kingdom.

If the letters are not used during the term of the prime minister who wrote them, they are destroyed unopened after that person leaves office, so that their content remains unknown to anyone except the issuer.