HMS M1 (Lt Cdr A M Carrie) sailed from Plymouth at 0300 on 12 November 1925 in company with HMS M3, to take part in an exercise off the South Devon Coast designed to simulate an attack by a group of cruisers on a submarine-escorted troop convoy. She rendezvoused with HMS L17, L22 and L23. The exercise called for L17 to scout five miles ahead of the convoy whilst L22 and L23 shadowed the convoy to starboard and M1 and M3 shadowed to port.

By 0645 all the submarines were in station. After sighting ‘enemy’ cruisers, M3 and M1 dived. M1 was observed broaching several times to simulate gun action, but soon afterwards the other submarines lost contact with her. M1 was sighted once more by the minesweeper HMS NEWARK but afterwards disappeared.

A clue to the mystery of her loss was found when the Swedish Collier SS Vidar berthed at Stockholm on 20 November and damage to her bow was discovered. She had been on passage through the English Channel from Cardiff on 12 November at about the same time and in the same area the exercise was taking place. She had been shaken by two heavy blows forward.

The Captain of Vidar had assumed that they were ‘submarine bombs’ used in the exercise and had proceeded with his passage. When dry docked in Sweden, it was later established that paint found on Vidar‘s stem was identical to that which had coated M1.

On the 29 September 1967, M1’s wreck was discovered unexpectedly by a salvage vessel searching off Start Point on the South Devon coast for the wreck of a torpedoed ship.