The UK government is currently led by the Labour Party.
This means that Tony Blair - as the leader of the Labour Party - is also the Prime Minister.
This will explain what happens after Mr Blair resigns, according to Labour Party rules.
If the serving Prime Minister belongs to another political party, there are different rules.
- Tony Blair announces he is to resign as Labour leader and Prime Minister. He will stay in the job until a replacement is chosen.
- Labour's National Executive Committee - the organisation that oversees the party and how it makes policies - meets to decide the timetable for choosing someone to take over from Blair and his deputy leader, John Prescott.
- The contest to decide who replaces them will last at least seven weeks.