The Sterling submachine gun is a British submachine gun (SMG). It was tested by the British Army in 1944–1945, but did not start to replace the Sten until 1953. A successful and reliable design, it remained standard issue in the British Army until 1994, when it began to be replaced by the L85A1, a bullpup assault rifle.

In 1944, the British General Staff issued a specification for a new submachine gun to replace the Sten. It stated that the new weapon should weigh no more than six pounds (2.7 kg), should fire 9×19mm Parabellum ammunition, have a rate of fire of no more than 500 rounds per minute, and be sufficiently accurate to allow five consecutive shots (fired in semi-automatic mode) to be placed inside a one-foot-square (30 cm × 30 cm) target at a distance of 100 yd (91 m).

To meet the new requirement, George William Patchett, the chief designer at the Sterling Armaments Company of Dagenham, submitted a sample weapon of new design in early 1944. The first Patchett prototype gun was similar to the Sten insofar as its cocking handle (and the slot it moved back and forth in) was placed in line with the ejection port though it was redesigned soon afterwards and moved up to a slightly offset position. The army quickly recognised the Patchett's significantly increased accuracy and reliability compared to the Sten and ordered 120 examples for trials. Towards the end of the Second World War, some of these trial samples are rumoured to have been used in combat by airborne troops during the battle of Arnhem and by special forces at other locations in Northern Europe where it was officially known as the Patchett Machine Carbine Mk 1. For example, a Patchett submachine gun (serial numbered 078 and now held by the Imperial War Museum), was carried in action by Colonel Robert W.P. Dawson while he was Commanding Officer of No. 4 Commando, during the attack on Walcheren as part of Operation Infatuate in November 1944. Because the Patchett/Sterling can use straight Sten magazines as well as the curved Sterling design, there were no interoperability problems.

After the war, with large numbers of Sten guns in the inventory, there was little interest in replacing them with a superior design. However, in 1947, a competitive trial between the Patchett, an Enfield design, a new BSA design and an experimental Australian design was held, with the Sten for comparison. The trial was inconclusive but was followed by further development and more trials. Eventually, the Patchett design won, and the decision was made in 1951 for the British Army to adopt it. It started to replace the Sten in 1953 as the "Sub-Machine Gun L2A1". Its last non-suppressed variation was the L2A3 but the model changes were minimal throughout its development life.

The price of an L2A3 in 1955 was £19.50, which is about £443 when adjusted for inflation.

Sterling submachine guns with minor cosmetic alterations were used in the production of the Star Wars films as props for the E-11 blaster rifle used by Imperial Stormtroopers. More drastically altered Sterlings were used as DH-17 blaster pistols in the hands of Rebel Alliance soldiers, though this weapon was mostly depicted via resin replicas that were cast from a "hero" prop and only a few live-firing Sterlings were modified compared to those used as E-11s. The E-11 itself would later come to be depicted by non-firing Sterling replicas, namely Model Gun Corporation replicas, which were used in Return of the Jedi and S&T Sterling L2A1 airsoft guns which have been used from Rogue One onwards.^{[better source needed]}

The Sterling submachine gun is constructed entirely of steel and plastic and has a shoulder stock that folds underneath the weapon. There is an adjustable rear sight, which can be flipped between 100 and 200 yard settings. Although of conventional blowback design firing from an open bolt, there are some unusual features: for example, the bolt has helical grooves cut into the surface to remove dirt and fouling from the inside of the receiver to increase reliability. There are two concentric recoil springs which cycle the bolt, as opposed to the single spring arrangement used by many other SMG designs. This double-spring arrangement is intended to give improved reliability when firing 9mm ammunition types other than the British '2Z' standard, which typically have different propellant charges and bullet weights.

The Sterling's double-stack, double-feed box magazine was designed in 1946 by George Patchett. While the original Patchett gun was intended to take Sten or Lanchester magazines, these magazines' poor reliability led Patchett to initially redesign the Sten magazine with a roller platform to reduce friction, and then to construct a new double-feed magazine that implemented the earlier roller platform, a stamped metal construction, and a curved magazine body, allowing the 9×19mm round to feed more reliably. The bolt feeds ammunition alternately from the top and bottom of the magazine lips, and its fixed firing pin is designed so that it does not line up with the primer in the cartridge until the cartridge has entered the chamber. In its final iteration, the magazine uses a four-piece construction with scalloped, spot-welded edges and positioning notches. While originally intended to hold forty rounds, concerns over the magazine's ability to fit into the ammunition pouch of the then-new 1944 Pattern Web Equipment led the magazine to be shortened to its final capacity of thirty-four rounds; when introducing the Mk 7 Pistol variant in 1983, Sterling produced magazines with capacities of ten and fifteen rounds for that weapon, as well as introducing "twin-stacked" versions of these and the original 34 round magazine. The Sterling magazine is said to be one of the best ever designed. While Sterling provided magazines for British Armed Forces weapons, with these being designated as "Magazine, 9mm, L1A1", cost issues led the Ministry of Defence to pursue the design and production of a non-Sterling L1A2 version which used a stamped and electrically seam-welded two-piece construction with a singular positioning lug and was 9.8" long down the rear spine compared to the 9.6" of Sterling-produced magazines. Production Sterlings retain the earlier Patchett gun's compatibility with Sten and Lanchester magazines; while the Lanchester gun is theoretically capable of using Sterling magazines, the same cannot be said for the Sten gun since a Sterling magazine intrudes 3⁄32 inch further into the bolt way than a Sten magazine and so attempting to fire a Sten gun with a Sterling magazine fitted would cause the breech block to foul the rear of the magazine, while attempting to counteract this by withdrawing the magazine by 3⁄32 inch would merely result in the magazine's top round being out of alignment with the breech block by 3⁄16 inches. This issue is not present on Sterling guns fitted with Sten magazines; while the magazine will be 3⁄32 inch further away from the breech block's centre line, the top round remains in line with the breech and so can be reached by the Sterling's more robust feed horns. The Sterling gun and its magazine were purportedly designed this way in order to prevent a situation where, instead of buying the actual Sterling gun, prospective users might only buy its magazines and then use these with their pre-existing Sten guns.