The British preemptively dispersed shipping from the harbours before the attacks due to advance warning from intelligence in March 1942, and air reconnaissance during the raid.

Upon Japan's entry into the Second World War, the air defences on Ceylon consisted of only four obsolescent three-inch anti-aircraft guns at Trincomalee.

[1] With the loss of Singapore in February 1942, British planners identified the island as essential to the defence of India and Allied lines of communication through the Indian Ocean.

[3] The threat of Japanese carrier-borne air attack was recognized from the examples of Pearl Harbor in December 1941 and Darwin in February 1942.

[7] 222 Group also received Consolidated PBY Catalina flying boats, which spotted and shadowed the Japanese fleet during the raid.

The station became operational at the Royal Colombo Golf Club – about 6 miles (9.7 km) north of Ratmalana – on 25 March, and was connected by telephone to the No.

The British defensive plan was based on assessments from the Far East Combined Bureau (FECB), which expected a "C day" of 1 April.

The Eastern Fleet, led by Vice Admiral Sir James Somerville, sortied on 30 March and deployed in a patrol area 100 miles (160 km) south of the island.

[14] With no sightings of the Japanese, the Eastern Fleet retired late on 2 April toward Addu Atoll to refuel at Port T, about 600 miles (970 km) southwest of Ceylon.

Later that day, the crew decided to repeat an assigned leg that took them to the southern-most point of their patrol; the repetition was conducted for navigational purposes.

[18] At 16:00, at the southern-most point of the repeated leg, QL-A spotted the Japanese fleet on the southern horizon about 360 mi (310 nmi; 580 km) south-east of Ceylon.

[19] Catalina FV-R from 205 Squadron RAF, piloted by Flight Lieutenant "Jock" Graham, took off at 17:45 and took over shadowing the Japanese fleet.

The last reported Japanese ships 110 miles (180 km) and 195 degrees from Dondra Head, the southern tip of Ceylon.

The Catalina was shot down by Japanese fighters about 90 minutes after making the final report, with the loss of the entire crew.

[8] Within an hour of QL-A's report, D’Albiac met with his subordinates to discuss an anticipated Japanese strike after dawn.

Cornwall and Dorsetshire left Colombo late on 4 April to rejoin the Eastern Fleet,[23] part of which had started sailing back to Ceylon from Port T after the QL-A report.

[15] At 05:45 on 5 April, Catalina BN-L of 240 Squadron RAF, piloted by Flight Lieutenant Bradshaw, took off to shadow the Japanese fleet.

[25] Shortly after 06:40, Catalina BN-L spotted four Japanese aircraft flying north but misidentified them as Hurricanes and made no report.

[21] The strike group approached Colombo from the sea to the south-east;[7][21] this course prevented detection by coast watchers and the 803 NAS line patrol.

[6] 19 Vals from Shokaku attacked the harbour around 07:50; they ultimately sank the armed merchant cruiser HMS Hector and damaged the submarine depot ship Lucia.

[9] The Shokaku Vals fully occupied 258 Squadron RAF, allowing the remaining bombers to attack the harbour and port unimpeded by British fighters.

[6] Six Fairey Swordfish from 788 Naval Air Squadron were also shot down; they were transferring from China Bay to Colombo for a strike on the Japanese fleet and arrived during the attack.

The distance travelled by Japanese aircraft between the edge of that radius to Ratmalana was 54 mi (87 km), and could be covered by a Zero at cruise speed in about 17 minutes.

[10] Birchall was awarded the Distinguished Flying Cross for the sighting made by Catalina QL-A on 4 April,[30] and was called the "Saviour of Ceylon" by Canadian media.