Who was the longest reigning prime minister of Britain in the 20th century?

Answer 1

Currently, David Cameron is the 37th longest serving Prime Minister of the United Kingdom, with 2 years and 92 days of service on August 20th 2012. If Cameron remains in office until March 27 2013, he will have been in power for longer than his immediate predecessor Gordon Brown.

In order to become the longest-running Prime Minister in UK history, Cameron would need to be in power until March 22 2031, at which point he would replaced Sir Robert Walpole as the longest-serving PM. This is highly unlikely to happen in the modern age - it would require the Prime Minister to win the 2015, 2020, 2025 and 2030 general elections (assuming no early dissolutions). No British leader of the modern era has been able to win more than three general elections back to back; no party has won more than four. Furthermore, no government has waited five years between elections and increased its majority since the mid-19th century.

A more realistic goal would be to become the longest-serving Prime Minister of the modern era. In order to do this, Cameron would need to serve until December 7 2021, at which point his length in office would have surpassed that of Margaret Thatcher's tenure. Tony Blair came close to his achievement himself, falling 1 year and 153 days short.

Answer 2

Margaret Thatcher, who was in office from the 4th of May 1979 until the 28th of November 1990.

Answer 3

Margaret Thatcher, who was Prime Minster from May 1979 until November 1990.