On 17 June 1940, HMT Lancastria was sunk by a German bomber while evacuating troops from St Nazaire; over 9,000 troops were packed on board. The exact number of soldiers who died that day will never be known, though even the lowest estimates rank this as the worst British maritime disaster in history, with losses exceeding those of the Titanic and Lusitania combined. This talk attempts to explain why so many who were lost will never be accounted for.
http://www.nationalarchives.gov.uk/podcasts/loss-of-lancastria.htm