Getting used to being prime minister takes some time. Getting used to being in charge of the nuclear codes, it seems, takes even longer.
A former Cabinet secretary has told of the "shock" new prime ministers feel when they have to write orders to Britain's nuclear submarines.
Former chief civil servant Lord Butler told the BBC Radio 4 programme Day One in No 10 about the reaction of both John Major and Tony Blair when he briefed them on their responsibility for deciding how and when to use the country's nuclear weapons, as they took office.
Here are the wise words of Lord Butler: "They reacted as you'd expect any human being to react: soberly, shocked if they hadn't realised before that that was one of the things that they would have to do at the start of their administration. So I would say it was a shock and a sobering one."
New premiers are briefed by the Cabinet secretary on what their options are regarding the use of the UK's four Trident submarines in case the country's government is destroyed.
They are then left to write the secret letter to Trident commanders alone, "wrestling with their own beliefs and conscience," according to Lord Butler.
The letters are sealed and sent to the submarine commanders and are top secret.
"I mean they are clearly really secret because the whole point of a nuclear deterrent is who your enemy doesn't know what he may incur if he attacks you, and so these are highly secret things, and only one person, who is the initiator of them, knows what the orders are, and that is the prime minister."
Let's hope Cameron's letter-writing skills are up to scratch. He won't want to complete this one at the end of a long day in power.