The action off Lerwick (/ˈlɛrɪk/, "Lerrick") was a naval engagement on 17 October 1917 fought in the North Sea during the First World War. The German light, minelaying cruisers SMS Brummer and Bremse attacked a westbound convoy of twelve colliers and other merchant ships and their escorts, part of the regular Scandinavian convoy. The two escorting destroyers and nine neutral Scandinavian ships were sunk off Shetland, Scotland.

Admiralty code breakers had uncovered the call signs of Bremse and Brummer and by direction finding knew that they had sailed from Wilhelmshaven to Lister Tief (Lister Deep) north of Sylt but an operation as far north as the Scandinavian convoy collier route was not anticipated by the British naval commanders because the Admiralty failed promptly to pass on the information.

Both of the British destroyer commanders were commended for their bravery, though some members of the Admiralty felt that by leaving the convoy to engage a superior enemy force, they had left the merchant ships open to attack. Other British ships did not receive reports of the attack until late afternoon, were not in a position to intercept and the German cruisers returned safely to port. The British called the attack on neutral vessels, giving no time for the crews to abandon ship, an outrage.

The Germans denied allegations that their ships had fired on survivors in the water, despite the evidence of gunshot wounds to some crewmembers. Admiral David Beatty, the commander-in-chief of the Grand Fleet, severely criticised the Admiralty for failing to pass on information derived from the code-breakers of Room 40.

The German U-boat campaign inflicted serious losses on neutral shipping but British counter-measures along the east coast of Britain and in the North Sea, including escorted convoys, managed to limit the losses of Scandinavian merchant ships. British losses continued unabated and in the last three months of 1916 losses of 618,000 gross register tons (GRT) were matched by only 220,000 GRT of new ships. From January to March 1917, 912,000 GRT were lost for a replacement of 326,000 GRT. Neutral shipping was an important addition to the British merchant fleet but in the last quarter of 1916, non-British shipping losses, excluding those of the Central Powers were 1, 159,000 GRT and in the next three months were 1,619,000 against new building of 587,000 GRT (mostly US construction). British ships were being fitted with defensive armament, often a 4.7-inch gun with a naval crew. Neutrals were afraid to arm their ships and compromise their status as non-combatants, making them more vulnerable than the British ships.

In February 1917, the British government took soundings with several neutral governments over the purchase of their merchant fleets but the suggestion was spurned. In negotiations with the Norwegian government about the resumption of coal imports from Britain, the Norwegians were so desperate for coal that they offered the merchant fleet if the matter could be carried off without German reprisals. The suggestion was all the more compelling to the Norwegians because the British could requisition their ships with no provision for any supplies to Norway. In June 1917, about 300,000 people protested against rising food prices, 40,000 in the capital, Kristiania. The Norwegian prime minister, Gunnar Knudson, thought that another coal shortage would cause mass unemployment, social unrest and possibly revolution. It was suggested that rather than sell the fleet, the Norwegians should charter it and that armed British ships be substituted on the routes suffering the worst losses from German U-boats. The Norwegian Shipowners' Association (Norges Rederiforbund) agreed to the Tonnage Agreement in which the transport of coal to Norway would be assured by British charter or requisition of the Rederiforbund ships. The connivance of the Norwegian government in the arrangement would be camouflaged by being made by the Rederiforbund and the Ministry of Shipping. The British gained 130 freighters of 200,000 GRT and the Norwegians 250,000 long tons (250,000 t) of coal per month.

In April 1917, the British began shipping coal north from the Humber Estuary to Lerwick in the Shetland Islands and then across the North Sea to Norway. Beginning in autumn 1914, six minesweeping trawlers sailed from each port on the east coast just before dawn and swept a channel 800 yd (730 m) wide and 200 nmi (370 km; 230 mi) long, the sweep being repeated late in the day. The swept channel was extended northwards to Scotland and the Orkney Islands. The monthly quota of 250,000 long tons (250,000 t) required daily convoys from Lerwick, most being neutral Scandinavian ships, usually escorted by two British destroyers from a pool of eight detached from the Grand Fleet, supported by several armed trawlers. To 1 July, 351 ships made the eastbound journey from Lerwick and 385 the voyage westwards from Norway, an average of 368 round trips a month, for the loss of twenty ships. In July, ten ships were sunk by the attacks of SM U-67, SM U-60, SM U-94, SM U-52 and SM UC-55, from 454, a loss of 2.2 per cent. Seven depth charge attacks were made by escort ships to no effect and HMS Itchen was torpedoed while escorting colliers northwards up the British coast to Lerwick. In anticipation of the added difficulty of escorting colliers in the autumn, winter and spring, several cruisers were sent to the Humber. The Scandinavian convoys had been a considerable success but had to sail in waters where they were vulnerable to German surface vessels for most of the journey. The return voyage began in Bergen in Norway, a neutral port where sailings could be observed by Germans and the use of neutral ships made it impossible for the British to keep convoy procedures and sailings secret.

Earlier in 1917, the Admiralty anticipated that U-boats would soon return to Germany. In early October 1917 another attack was planned. U-boats sailed between two minefields to the east of the Dogger Bank and the Outer Silver Pit, a funnel in the North Sea 315 nmi (362 mi; 583 km) long, 85 nmi (98 mi; 157 km) wide at the north end, narrowing to 45 nmi (52 mi; 83 km) at the southern end. A barrage of underwater mine nets were to be laid at the narrow end of the funnel. At the wide (northern) end, four submarines mounted a standing patrol and further south, two destroyer leaders, fourteen destroyers and a yacht patrolled between the Moray Firth and the Firth of Forth. Net drifter fishing boats, were to place nets in the funnel between the Firth of Forth and Flamborough Head. Four destroyers and sixteen trawlers, commanded from the yacht, HMY Goissa. The southernmost end of the funnel, where U-boats had to skirt the Dogger Bank minefields, was to be patrolled by Harwich Force. To maintain the standing patrols required between 15 and 29 destroyers and flotilla leaders, with 18 destroyers from Harwich Force, beginning on 27 September.