HMS Cygnet was a C-class destroyer built for the Royal Navy in the early 1930s. The ship was initially assigned to the Home Fleet, although she was temporarily deployed in the Red Sea during the Abyssinia Crisis of 1935–36. Cygnet was sold to the Royal Canadian Navy (RCN) in late 1937 and renamed HMCS St. Laurent. She was stationed on the west coast of Canada when World War II began in September 1939, and had to be transferred to the Atlantic coast for convoy escort duties. She served as a convoy escort in the Battle of the Atlantic and participated in the sinking of two German submarines. The ship was on anti-submarine patrols during the invasion of Normandy, and was employed as a troop transport after VE Day for returning Canadian servicemen. St. Laurent was decommissioned in late 1945 and scrapped in 1947.

Cygnet displaced 1,375 long tons (1,397 t) at standard load and 1,865 long tons (1,895 t) at deep load. The ship had an overall length of 329 feet (100.3 m), a beam of 33 feet (10.1 m) and a draught of 12 feet 6 inches (3.8 m). She was powered by Parsons geared steam turbines, driving two shafts, which developed a total of 36,000 shaft horsepower (27,000 kW) and gave a maximum speed of 36 knots (67 km/h; 41 mph). Steam for the turbines was provided by three Admiralty 3-drum water-tube boilers. Cygnet carried a maximum of 473 long tons (481 t) of fuel oil that gave her a range of 5,500 nautical miles (10,200 km; 6,300 mi) at 15 knots (28 km/h; 17 mph). The ship's complement was 145 officers and men.

The ship mounted four 45-calibre 4.7-inch Mk IX guns in single mounts, designated 'A', 'B', 'X', and 'Y' from front to rear. For anti-aircraft (AA) defence, Cygnet had a single QF 3-inch 20 cwt AA gun between her funnels, and two 40-millimetre (1.6 in) QF 2-pounder Mk II AA guns mounted on the aft end of her forecastle deck. The 3-inch (76 mm) AA gun was removed in 1936 and the 2-pounders were relocated to between the funnels. She was fitted with two above-water quadruple torpedo tube mounts for 21-inch torpedoes. Three depth-charge chutes were fitted, each with a capacity of two depth charges. After World War II began this was increased to 33 depth charges, delivered by one or two rails and two throwers.

The ship was ordered on 15 July 1930 from Vickers-Armstrongs, Barrow-in-Furness under the 1929 Programme. Cygnet was laid down on 1 December 1930, launched on 29 September 1931, as the 14th ship to carry the name, and completed on 1 April 1932.

After the ship commissioned on 9 April 1932, she was assigned to the 2nd Destroyer Flotilla of the Home Fleet. Cygnet spent a lot of time in dockyard hands during her first two years of service. She was repaired at Devonport in November 1932 – January 1933, March–May, July–August and November 1933 – January 1934 before deploying to the West Indies with the Home Fleet between January and March 1934. The ship required more repairs upon her return in April–May and then a refit from 25 July to 31 August 1934. Cygnet was detached from the Home Fleet during the Abyssinian Crisis, and deployed in the Red Sea from September 1935 to April 1936. The ship returned to the UK in April 1936 and refitted at Devonport between 20 April and 18 June before resuming duty with the Home Fleet. In July–August she was deployed for patrol duties off the Spanish coast in the Bay of Biscay to intercept shipping carrying contraband goods to Spain and to protect British-flagged shipping during the first stages of the Spanish Civil War.

Together with her sister HMS Crescent, Cygnet was sold to Canada on 20 October 1936 for a total price of £400,000. She was refitted again to meet Canadian standards, including the installation of Type 124 ASDIC, and handed over on 1 February 1937. The ship was renamed as HMCS St. Laurent and commissioned into the RCN on 17 February. St. Laurent was assigned to Halifax, Nova Scotia and arrived there in May. She remained there for a year before she was transferred to Esquimalt in 1938. The ship remained there until she was ordered to the East Coast on 31 August 1939, arriving at Halifax on 18 September. St. Laurent escorted local convoys while based there, including the convoy carrying half of the 1st Canadian Infantry Division to the UK on 10 December. The ship was ordered to Plymouth on 24 May 1940 and arrived there on 31 May. Upon arrival, the ship's rear torpedo tube mount was removed and replaced by a 12-pounder AA gun and the 2-pounders were exchanged for quadruple Mark I mounts for the QF 0.5-inch Vickers Mark III machine gun.

On 9 June, St. Laurent was ordered to Le Havre, France to evacuate British troops, but none were to be found and the ship evacuated a small group of French soldiers further up the coast on 11 June. The ship was taken under fire by a German artillery battery near Saint-Valery-en-Caux, but she was not hit and Lieutenant Commander H.G. DeWolf, the ship's captain, ordered her to return fire although no results were noted. After returning to England, St. Laurent escorted several troop convoys on the last legs of their journeys from Canada, Australia and New Zealand in mid-June and was assigned to escort duties with Western Approaches Command afterwards.

On 2 July, whilst escorting the British battleship Nelson, St. Laurent received word that the unescorted British passenger ship SS Arandora Star had been torpedoed by U-47, about 125 nautical miles (232 km; 144 mi) northeast of Malin Head, Ireland. Arriving some four and a half hours after the ocean liner sank, the ship rescued 857 survivors, including German and Italian prisoners of war. Together with the British sloop Sandwich, she badly damaged the German submarine U-52 whilst defending Convoy HX 60 on 4 August. On 2 December, St. Laurent rescued survivors from the armed merchant cruiser HMS Forfar that had been torpedoed and sunk by U-99 as well survivors from the British oil tanker Conch.