The idea of using a small carrier for what the RN described as 'trade protection' had been brought up during the interwar period, though it was not taken further, largely due to opposition from the Air Ministry. At the outset of war, aircraft carriers were deployed as part of ASW hunting groups. This saw little success, and led to the loss of HMS Courageous. However, the threat of the German FW-200 Condor patrol aircraft, as well as submarines, led to the development of the escort carrier. These small carriers, mostly converted from merchant hulls, accompanied convoys and, guided by Enigma decrypts, hunted for German submarines. They supplemented by MAC ships, merchant ships with a flight deck bodged on top.

The first suggestion for a light carrier for the protection of trade came in 1926. This proposal was for a seaplane carrier, launching its aircraft by catapult. There was no need for a landing deck, as the aircraft could land on the sea and be hoisted back aboard by crane. The suggestion came from the Admiralty, and was passed to the Air Ministry. It ended there, with no design being produced. In 1932-34, the Admiralty considered small carriers converted from merchant ships. This proved the feasibility and utility of the concept, especially when applied to passenger liners. In February 1935, design studies were produced based on two liners, the 20,000 ton Winchester Castle and the Waipawa of 12,500 tons. The former was preferred, as Waipawa was too small to be effective. In October 1935, the Admiralty proposed the construction of a force of 14,500 ton carriers, carrying 15 aircraft, to be used as convoy escorts and as parts of ASW hunting groups. However, the Air Ministry was firmly against this. Group Captain Arthur Harris (who would later direct the RAF's bomber offensive against Germany) wrote a vituperative refusal, arguing that the Admiralty's ideas would divert aircraft and resources away from where they would be more useful. As the Air Ministry was, until 1939, responsible for naval aircraft procurement, their refusal to cooperate spelled the end of this idea. There was no point building carriers for which there would be no aircraft. There were other issues to face, though, especially the limits on carrier tonnage enforced by the Washington Treaty of 1922. The idea of using liners converted in the event of war remained, with a shortlist of five ships, including Winchester Castle, being drawn up in 1937. The idea would be brought up at trade protection conferences held in 1937-38, but would ultimately be rejected. The lack of aircraft was one aspect of this rejection. There was also a feeling that there would be too much need for merchant shipping to spare any ships for conversion - and that conversion might take too long in the event of a short war. Finally, it wasn't clear how useful they would be. The British expected the German submarine offensive to take place close to the UK, in the North Sea and the Western Approaches. These were areas that could easily be covered by land-based aircraft, removing the need for an aircraft carrier. As a result of this, no practical work had been done on escort carriers before the start of the war.

Once the war started, the RN deployed threee carrier task forces as 'hunting groups'. Based around larger carriers, they patrolled the waters west of the UK. They saw little success. The first task force was centred around HMS Ark Royal, and operated in the North-West Approaches. On the morning of the 14th September, Ark Royal received a distress signal from a merchant ship, the Fanad Head. She turned into the wind to fly off three Skua dive-bombers. Doing so allowed the U-boat U-39 to move into a position to attack, firing a spread of torpedoes. Fortunately, all missed, with one exploding in the carrier's wake. Ark Royal's destroyer escort counterattacked, sinking U-39. Meanwhile, the Skuas had located the Fanad Head, and found the U-boat that had attacked her (U-30) on the surface. They attacked, but the blast from their bombs destroyed two of the aircraft. No damage was done to the U-boat by this attack, or by the follow-up attack by six Swordfish aircraft. Three days later, though, another carrier would not be so lucky. Courageous, operating in the South-West Approaches, had received a distress signal from another merchant. Two of her four destroyer escorts were detached to go to her assistance. This made an opening for U-29 to close to attack, made easier by Courageous turning to fly off aircraft to patrol. U-29 scored two hits on Courageous, sinking her in just under 30 minutes. She went down with 518 of her crew, including a significant number of her aircrew. The final carrier, Hermes, also operated in the South-West Approaches. Her patrols were relatively uninteresting, with only one U-boat being sighted by her aircraft. The patrols were cancelled on the 18th September. This is often thought to be the result of pressure by the Cabinet on the Navy after Courageous' loss. In addition to this, the patrols are generally regarded as a complete failure, representative of an RN that was overly focused on the offensive through hunting, rather than defensive actions through the convoy system. Malcolm Llewellyn-Jones has, however, made a cogent argument that the patrols were intended to be a temporary measure, covering ships that had left harbour before the convoy system was introduced. There is also evidence that the RN did not regard these patrols as the total disaster they are often portrayed as. Courageous' loss was seen as the cost of doing business, as well asthe result of poor tactical use of her air and surface forces, and the lack of an effective air ASW weapon. Commanders understood that aircraft could be used to force a U-boat to submerge, effectively fixing her in place, for a destroyer to find and kill her. This was a much more efficient way of sinking submarines than waiting for them to attack submarines. Revised tactical guidance was produced about a month after the loss of Courageous, but the carriers were seen as more useful in hunting for surface raiders, especially in the South Atlantic.

During the remainder of 1939, and the first half of 1940, the German submarine offensive followed the pattern that the pre-war RN had expected. Subs operated close to Britain, within the range of land-based aircraft. There was, therefore, no reason to build light ASW carriers. The Ministry of War Transport (MoWT) was loath to release merchant shipping, and shipbuilding resources were too scarce to devote to conversions. The fall of France and Norway in the late spring and early summer of 1940 changed this. With these bases, U-boats could operate deep into the Atlantic. The FW-200 Condor patrol aircraft, flying from bases in France, could locate convoys in the eastern Atlantic, vectoring in U-boats and attacking with bombs. The RN had no counter to the FW-200 immediately available. Two avenues were explored. The first was the CAM ship (or Fighter Catapult Ship, for ships in RN service). This mounted an obsolete or worn-out fighter, usually a Sea Hurricane or Fulmar, on a catapult on a merchant ship. When an Condor approached the convoy with the CAM ship, the fighter could be launched to attack. The pilot would then have to ditch, bail out or land ashore, as the CAM ship had no landing-on deck. The first CAM ships entered service in March 1941, and a total of 40 ships were converted (plus HMS Pegasus, a WWI-era seaplane carrier that had been used for catapult trials in the interwar period). They scored a total of seven kills, plus four aircraft damaged, with several more aircraft being driven away from the convoys. The idea was unsuccessful, and most CAM ships were converted back to normal merchants in 1943.

More successful was the escort carrier. In the autumn of 1940, Matthew Slattery, Director of Air Material at the Admiralty, suggested that merchant ships be fitted with a simple flight deck and arrester wire system, allowing them to launch and recover fighters and ASW aircraft. The MoWT still refused to allow merchant ships to be converted away from carrying cargo, but the Admiralty was able to use ships already allocated to it. A range of design speciifcations were drawn up, for three classes of ships, designated A, B and C. Class A ships could carry 25 aircraft, with hangar space for 16, and had a top speed of 20 knots. Class B carried 15 aircraft with a hangar for 12, and had a top speed of 18 knots. Class C carried ten aircraft, with hangar space for four, and had a top speed of 16.5 knots. Meanwhile, the captured German merchant Hannover, transferred to the RN as the boarding ship Empire Audacity, was taken in hand for a conversion in January 1941. Her superstructure, funnels and masts were razed, and replaced by a flight deck. She had no hangar or island. Instead, her aircraft were placed in a deck park, and she was conned from a pair of platforms on either side of the flight deck, about 4ft below it. She had an air group of 8 Martlet fighters (Wildcats in US service). She entered service in June 1941, with her name reduced to Audacity to stop RN commanders thinking she was a merchant ship. During her brief service, she escorted four convoys to and from Gibraltar. During these, her fighters shot down five Condors, and helped surface escorts find and sink eight U-boats. Unfortunately, on her fourth, she was sunk by U-751, having been left outside the convoy's protective screen at night. Audacity's success opened the gateways to more escort carriers. While Audacity was being converted, the USN was carrying out a similar experiment with the USS Long Island, also converted from a merchant ship.

So there WERE escort carriers operating with Atlantic convoys. I heard of those used in pacific theatre.