'Klaxon I' was the British naval evacuation of the Allied troops from Namsos on the west coast of central Norway (2/3 May 1940).

Vice Admiral J. H. D. Cunningham, commanding the 1^st Cruiser Squadron, had departed Scapa Flow in the Orkney islands Group on 29 April with the intention of evacuating half of Major General A. Carton de Wiart’s 'Maurice' Force (Brigadier C. G. Phillips’s 146^th Brigade and Général de Brigade Marie Emil Antoine Béthouart’s French 5ème Demi-Brigade des Chasseurs Alpins) in three large French transport ships during the first night, and the other half during the second night in the heavy cruisers Devonshire and York, and the French light cruiser Montcalm.

There was a German air attack in the afternoon of 1 May, Devonshire and one of the transports just escaping hits. During the evening the ships encountered thick fog some 40 miles (65 km) from the intended rendezvous, namely the Kya Light, itself some 40 miles (65 km) short of Namsos. The operation for the night of 1/2 May was therefore cancelled by a message which reached the troops ashore at 21.30 and required the dispersal assembled troops, who just got back into position by daybreak. There was clear sky over Namsos itself, as was discovered by Captain the Lord Mountbatten’s force of Kelly, Maori and two other destroyers which managed to find their way to the port through the fog but were then compelled to put to sea again without embarking any of the troops as the arrival of better weather allowed the renewal of German air attacks. One of the destroyers suffered 23 casualties from a near miss as she hid in a fog bank which was just too low to cover her masthead.

The situation was critical as the Germans would inevitably become aware of what was afoot: the German air command at Trondheim had in fact already discovered and reported the start of the evacuation. Two other factors, neither of which was known to the commanders ashore, further exacerbated the situation: the two British fleet carriers available for support of Norwegian operations had in fact departed earlier in the evening of the same day, and the political situation prevented Prime Minister Neville Chamberlain from any further postponement of a statement on the progress of the war in Norway, which would reveal the completion of the parallel evacuation from Åndalsnes and inevitably point the Germans' attention to the similar move impending at Namsos.

Chamberlain’s statement was made during the afternoon of 2 May, much to the discomfiture of 'Maurice' Force in Namsos. It was de Wiart’s opinion that the shortness of the dark hours at this time of the year made the completion of the evacuation in one night desirable but also impractical. Cunningham had been warned right from the beginning by Admiral Sir Charles Forbes, commanding the Home Fleet, that the task might have to be accomplished in one lift, however, and Cunningham had therefore prepared an alternative plan on 30 April, when he knew that one French battalion had been removed in the store ships which had left Namsos on 29 April.

There were about 5,400 men to be evacuated: three transports could embark 1,700 men each at the stone pier, and the other 300 could be accommodated in York, receiving them from trawlers which would ferry the men out of the port. Rear Admiral J. G. P. Vivian, who had been in the port with the light anti-aircraft cruiser Carlisle, shared the ground force’s belief that two nights would be needed, but the transports were running short of fuel and Cunningham decided to try a single-night lift.

On the second night, that of 2/3 May, the squadron ran out of the fog 40 miles (65 km) from the Namsenfjord, where it divided. The admiral with two cruisers and four destroyers waited off the Kya Light while the three French transports (El Djezair, El Kantara and El Mansour), York and five destroyers under the command of Captain P. L. Vian went straight in, arriving off Namsos at about 22.30. Two transports loaded at the stone quay, the other two large ships were loaded from destroyers and trawlers, and the destroyer Afridi took the last parties on board at 02.20.

It was a close-run thing, for the Bangsund bridge was not demolished until after 24.00, leaving the rearguard to cover 10 miles (16 km) to the port. Full light was close by 01.30, but the trucks arrived soon after this. The German forces had in fact received sufficient notice of the undertaking, which began at a point well beyond the ability of the British to provide fighter cover, considerably more risky than the Åndalsnes run of the previous two nights. German aerial reconnaissance thus spotted the later groups in the convoy at 04.30, these comprising two large French transports each escorted by one of Cunningham’s cruisers and destroyers. They were attacked on five occasions between 08.45 and 15.30, by which time the first air escort of one Short Sunderland flying boat was approaching the head of the convoy, at distances ranging from 140 to 220 miles (225 to 355 km) off the German-occupied airfield of Værnes.

Anti-aircraft fire from the warships and from the French transports under Contre-amiral Jean Emmanuel Cadart destroyed two or three dive-bombers out of the 50 or so aircraft which delivered the attacks, and the ships eventually moved into line ahead formation for better mutual support, with Carlisle last astern. But the third attack caused the French destroyer Bison to catch fire, and the ship was sunk by British destroyers after they had taken aboard the surviving member of the French crew. This caused some delay to the destroyers concerned, and Afridi was hit by two bombs at 14.00 while returning to the squadron. She eventually capsized with the loss of about 100 men.