Boris Johnson needs to continue as prime minister until November 20 to avoid becoming the shortest-serving PM in UK history.
On that date he will overtake George Canning, who clocked up 118 days as prime minister before his death in 1827, and who holds the record for the shortest time in office.
Mr Johnson is currently on 41 days.
George Canning served as Tory prime minister between April and August 1827.
He is one of a small number of prime ministers to die while still in office.
Canning is also famous for taking part in a duel with a fellow cabinet minister.
The event took place at 6am on September 21 1809 on Putney Heath in south London, and involved Canning, then foreign secretary, drawing a pistol against minister of war Viscount Castlereagh.
Castlereagh believed Canning had been plotting to get him sacked and challenged him to a duel.
Canning failed to shoot his colleague and in return was wounded by Castlereagh in the thigh.