HMS A1 (Lt L O Mansergh) was lost on 18 March 1904 with all hands, 5 miles off the Nab light vessel in the Solent. A1 had been running in for a simulated attack along with some Holland-class submarines (including HOLLAND 2 and HOLLAND 3) on the cruiser HMS JUNO which was returning to Portsmouth after participating in manoeuvres with the Channel Squadron. Mansergh did not see the SS Berwick Castle approaching from his starboard quarter and the two vessels collided. Whilst the pressure hull was undamaged, A1’s conning tower was holed and the periscope and upper lid were damaged resulting in the boat being completely flooded.
As a result of A1 flooding through the conning tower, a lower lid was fitted to all other ‘A’ boats and to all later classes of submarine.
A1 was the first submarine loss suffered by the Royal Navy.