Theresa May will be the 13th prime minister to serve under the Queen.
Here is a look back at those who have led Her Majesty's government during her 64-year reign.
:: Sir Winston Churchill was already in office when the Queen succeeded to the throne. The wartime prime minister served his second term from 1951 to 1955 after beating Clement Attlee in the general election. Churchill was forced to resign because of poor health in 1955.
:: Sir Anthony Eden became prime minister after the resignation of Churchill in 1955. He immediately called a general election and increased the Conservatives' majority. However, his term in office lasted just two years because of his controversial handling of the Suez crisis.
:: Harold Macmillan told the Queen that he had little hope his government would survive in office for more than six weeks when he became prime minister. The Conservative party was demoralised after the Suez crisis, but Macmillan turned the tables and eventually claimed that British people had "never had it so good". He resigned in 1963.
:: Sir Alec Douglas-Home's time in Number 10 was short lived. He spent just 363 days in office after losing the general election to Labour in 1963. The aristocratic peer was a member of the House of Lords when the Conservatives chose him to replace Macmillan. He renounced his peerage and won a by-election for the safe Kinross and Perthshire West Commons seat, but lost the election the following year.
:: Harold Wilson championed many social reforms during his time in office. As Labour leader, he won the general election in 1964 with a small majority which he increased significantly by the 1966 election. His government enacted an array of social changes - from housing and gender equality to pensions and education - but he was defeated in the 1970 general election. He took office for a second time in 1974 - initially with a minority government after a hung parliament. Wilson spent the next two years focusing on domestic policy - particularly social reforms which saw income tax for top earners hit 83%. He surprised the nation when he resigned in 1976.
:: Sir Edward Heath took the helm in 1970 promising to turn around the nation's fortunes. Britain was in the midst of economic decline, but unemployment continued to rise during his term. Heath tried to weaken the power of unions, but the Miners' Strikes during his term led to the three-day week. He lost the general election in 1974, although he served in the House of Commons until 2001.
:: James Callaghan had already held three major offices of state when he succeeded Wilson to be prime minister. The Labour leader had been Chancellor of the Exchequer, Home Secretary and Foreign Secretary when he took on the top job, but survived for just three years. He was undermined by workers' strikes during the "Winter of Discontent" and a motion of no confidence in Callaghan was passed by one vote.
:: Baroness Margaret Thatcher was Britain's first female prime minister and the longest-serving PM for over 150 years. The daughter of a shopkeeper, she led the Conservatives to victory in the 1979 general election. She was behind the radical, and controversial, privatisation schemes and gave local authority tenants the right to buy their homes. She resigned from office in 1990.
:: Sir John Major became Tory leader after Thatcher's resignation and went on to win the 1992 general election. Unlike many of his predecessors, Major left school at 16 and did not go to university. During his term in office, he helped to establish the Northern Ireland Peace Process and replaced the unpopular Poll Tax.
:: Tony Blair became the longest-serving Labour prime minister after he entered office with a landslide majority in 1997. As a champion of "New Labour", Blair's decade as leader saw him reform the House of Lords and introduce civil partnerships. His decision to back the American-led invasion of Iraq lost him supporters, and he resigned in 2007 after facing criticism over his continued support for President Bush over the Israel-Lebanon war.
:: Gordon Brown was chancellor before moving to Number 10 in 2007. The Scot implemented the world's first climate change act and introduced new powers for counter-terrorism police to share information, but his tenure in office was marred by the global financial crisis. He resigned in 2010 after the general election resulted in a hung parliament.
:: David Cameron was the youngest prime minister in almost 200 years when he formed a coalition government with the Liberal Democrats. A former shadow education secretary, Cameron was elected leader of the Conservatives in 2005 before coming into power in 2010. He won a majority in the 2015 general election but resigned as prime minister in the wake of the Brexit vote.
:: Theresa May will be Britain's second female prime minister when she takes the reins this week. Mrs May started her career at the Bank of England and became an MP in 1997. She held a variety of shadow cabinet positions before becoming Home Secretary when the Tories came into power.