Seaslug was a first-generation surface-to-air missile developed by Armstrong Whitworth (later part of the Hawker Siddeley group) for use by the Royal Navy. Its origins date back to the 1944 LOPGAP project. The missile entered operational service in 1961 and remained in use until the early 1980s, including during the 1982 Falklands War.

Designed to engage high-altitude targets such as reconnaissance aircraft or bombers before they could launch stand-off weapons, Seaslug was fitted only to the eight County-class destroyers, which were purpose-built around the system. The missile was fired in combat at an airborne target on a single occasion, by HMS Antrim during the Falklands War, but failed to hit its target, although it was later used against surface targets.

Seaslug's medium-range role was initially intended to be replaced by the long-range Blue Envoy missile. However, Blue Envoy was cancelled in favour of a new medium-range system, Sea Dart. Sea Dart entered service in 1973 aboard the Type 82 destroyer HMS Bristol (D23) and gradually replaced Seaslug as the County-class destroyers were withdrawn from service during the 1980s.

In 1943, the German Luftwaffe began the use of anti-shipping missiles and guided bombs in the Mediterranean Sea during Allied operations against Italy. These weapons were released outside of anti-aircraft gun range, which meant that naval operations lacking complete air superiority would be open to attack with no effective response from the ships. A solution for long-range anti-aircraft was required. On 16 March 1944 the first meeting of the "Guided Anti-Aircraft Projectile Committee", was held.

The Admiralty Signals Establishment (ASE), in charge of the Navy's radar development, was working on new radars featuring radar lock-on that allowed them to accurately track aircraft at long range. This was part of the LRS.1 fire-control system that allowed large dual-purpose guns to attack bombers at long range. A contemporary British Army project at Cossors, Brakemine, was working on a system to allow a missile to keep itself centred within a radar beam, a concept known today as beam riding. The Navy decided to combine the two concepts, using the LRS.1's Type 901 radar with a new missile that differed from Brakemine primarily in requiring longer range and being more robust for shipborne use.

In December 1944, GAP put out a Naval Staff Target for a new anti-aircraft weapon, capable of attacking targets at altitudes up to 50,000 ft (15,000 m) and speeds of up to 700 mph (1,100 km/h). This project was briefly known as LOPGAP, short for "Liquid Oxygen and Petrol Guided Anti-aircraft Projectile", but soon moved from petrol to methanol which made the "LOP" inaccurate.

The Fairey Aviation Company at this time was working on a missile project for the Ministry of Supply, Stooge. Stooge was more like an armed drone aircraft than a missile. It was flown to a location in front of the target and then cruised toward it until its warhead was triggered by the operator. It was designed primarily to defeat kamikaze attacks at short range. Its low speed and manual guidance meant it was not useful for interceptions outside the immediate area of the ship, and thus did not meet the need for a longer-ranged missile capable of dealing with stand-off weapons.

Accordingly, Fairey was ordered to stop work on Stooge in favour of LOPGAP. Development was slowed by the Air Ministry who were opposed to the project as it might take resources away from jet fighter production and a lack of urgency on the part of both the Admiralty and Ministry of Supply.