RMS Lancastria was a British ocean liner requisitioned by the UK Government during the Second World War. She was sunk on 17 June 1940 during Operation Aerial. Having received an emergency order to evacuate British nationals and troops from France, the ship was loaded well in excess of its capacity of 1,300 passengers. Modern estimates suggest that between 4,000 and 7,000 people died during the sinking — the largest single-ship loss of life in British maritime history.

An ocean liner is a type of passenger ship primarily used for transportation across seas or oceans. Ocean liners may also carry cargo or mail, and may sometimes be used for other purposes. Only one ocean liner remains in service today.