The prime minister is normally (though not always) the leader of the majority party, or the largest single party, in the House of Commons.
As well as being leader of the main governing party the prime minister is also leader of the government.
He or she is responsible for selecting the cabinet and chairing its meetings.
The prime minister has the power to choose the date of the general election and can ask the monarch for permission to dissolve Parliament at any time.
Sir Robert Walpole is generally acknowledged as having been Britain's first prime minister, holding the office between 1721 and 1742.
The prime minister is also first lord of the Treasury, head of the Civil Service and, effectively, head of the Armed Forces.