<P> Prince Charles is the longest serving heir - apparent, Duke of Cornwall, and Duke of Rothesay, with a tenure of 7004242180000000000 ♠ 66 years, 111 days, and Prince of Wales, with a tenure of 7004218560000000000 ♠ 59 years, 306 days . </P> <P> Margaret of Scotland, Countess of Kent was heiress - presumptive to William I of Scotland, Alexander II and Alexander III for a total of about 43 years, 14 days (her tenure as heiress - presumptive to Alexander II was also the longest single tenure at 26 years, 274 days). The longest - serving heir - presumptive (of either sex) was Robert II, who was heir - presumptive to Robert I and then David II for a total of 46 years, 353 days; he also served the longest single tenure (41 years, 260 days as heir - presumptive to David II). </P> <P> The shortest - reigning monarch was Lady Jane Grey who ruled for 9 days from 6 July until 15 July 1553 (although she was only proclaimed queen by the Lords of the Council on 10 July). Note: Jane's reign is disputed . </P> <P> The king with the shortest definitively known reign was Edgar the Ætheling who ruled for 2 months, 12 days in 1066 before submitting to William the Conqueror . Some records indicate that Sweyn Forkbeard reigned for only 40 days in 1013 - 4 . </P>

Who is the shortest-reigning monarch in british history