Queen Elizabeth II is currently battling a "heavy cold" that has forced the monarch to skip annual royal holiday celebrations, including Christmas and New Year services, giving rise to speculation that the longest reigning monarch might have to leave her royal duties due to health concerns.

She is the second woman to rule the United Kingdom of Great Britain and Ireland, which formed in 1801 after a unified Kingdom of Great Britain merged with the Kingdom of Ireland. The only other woman in that place was her great-great-grandmother Queen Victoria, who took the throne in 1837 when she was 18 and ruled until her death in 1901. She was the last monarch of Britain from the House of Hanover, and was succeeded by her son, King Edward VII, who belonged to her husband's House of Saxe-Coburg and Gotha.

Queen Victoria, who died at the age of 81, had four sons including Edward VII, and five daughters. She reigned for 63 years, seven months and two days, to become the longest-reigning British monarch till Elizabeth II surpassed her on Sept. 9, 2015.

Queen Elizabeth II took the throne in 1952 after King George VI's death. In Britain, the crown is normally passed from the monarch to the eldest son. As King George VI had no son, the crown passed to his elder daughter. However, in 2011, the Act of Settlement was changed before Prince George’s birth, to ensure succession would not be affected by gender of the child.

It was during the reign of Queen Anne, who was on the throne from 1702 to 1714, that the Treaty of Union was signed between the Kingdom of England (which already included Wales) and the Kingdom of Scotland, leading to the creation of the Kingdom of Great Britain. Anne took the throne in 1702 after the death of William III. She was succeeded by her second cousin George I of the House of Hanover as Anne died at the age of 49 without any surviving children despite 17 pregnancies.

It was during the reign of King George III that the Kingdom of Great Britain merged with the Kingdom of Ireland, becoming the country we know today. Below is a timeline of those who ruled United Kingdom since.

  • King George III (1760-1820)
  • King George IV (1820-1830)
  • King William IV (1830-1837)
  • Queen Victoria (1837-1901)
  • King Edward VII (1901-1910)
  • King George V (1910-1936)
  • King Edward VIII (1936-1936, abdicated)
  • King George VI (1936-1952)
  • Queen Elizabeth II (1952-present)