H.M.S. Seagull is one of the two minesweepers ordered to be built at Devonport Dockyard as part of the 1936 Naval programme. It was decided that Seagull should be built all-welded, whilst the sister ship Leda should be constructed in accordance with the normal practice, that is, mainly riveted. The occasion was to be utilized by obtaining a trustworthy comparison between the two methods of construction. So far as was practicable, the thicknesses of plating were left unaltered. The Seagull was launched on October 28, 1937, the launching weight was 313 tons, and the weight of the Leda was 338 tons. The welded vessel was commissioned and completed in 1938. Both ships underwent rough-weather trials in Faroe–Icelandic waters for about a fortnight in September 1938.