In the aftermath of Britain’s decision to leave the European Union, David Cameron has announced that he will step down as Prime Minister in October. This led a friend of mine to ask why he didn’t resign right away.
The answer is that the delay is necessary in order to give the Conservative Party time to choose a new leader. Once that happens, Cameron will formally tender his resignation to the Queen and advise her to invite the new Tory leader to form a government.
If Cameron resigned right away, it would put Her Majesty in an awkward position. Unlike America, there is no designated successor who can step up if the Prime Minister leaves office suddenly. This is because the choice of a Prime Minister is technically a prerogative act of the Crown, and it is considered inappropriate to prejudge the Sovereign’s decision. For many years, the Palace was reluctant to allow the appointment of a Deputy Prime Minister, lest it create an assumption that the individual would have a right to succeed to the premiership. As a result, the position of Deputy Prime Minister remains an informal one. It’s basically a courtesy title that has no constitutional significance.
If a Prime Minister were to leave office unexpectedly, the Queen might have to choose the next premier on her own. This has actually happened twice in recent memory (Harold Macmillan in 1957 and Alec Douglas-Home in 1963). On both occasions, Her Majesty had to get involved because a Conservative Prime Minister stepped down and the party didn’t have a mechanism for choosing a leader when in government. This forced the Queen to rely on the advice of a circle of Tory grandees, which was hardly ideal. Thankfully, the Tories eventually learned their lesson, so the chances of the Crown having to make a political decision are much slimmer.
 Vernon Bogdanor, The Monarchy and the Constitution (Oxford: Clarendon Press, 1995), 88.
 Bogdanor, 87-88.