'Catherine' was was a British unrealised plan for naval operations in the Baltic Sea to halt Germany’s import of Swedish iron ore in the ice-free summer months between May and September, in concert with 'Wilfred' to achieve the same result in the winter months between October and April through the ice-free Leads, and also to interdict Germany’s maritime trade with Finland and the USSR (September 1939/May 1940).

The driving force behind the concept was Winston Churchill, then the First Lord of the Admiralty, and the planning was entrusted to its proposed commander, Admiral the Lord Cork and Orrery. Cork had retired from active service before the start of World War II but nonetheless offered his services on the outbreak of hostilities, only to be told that there was nothing for him. On 21 September, however, Churchill called him to the Admiralty and provided him with a small staff with which to consider and plan a Baltic Sea offensive to take place by March 1940. The project was dear to Churchill’s heart, reminiscent as it was of a World War I scheme to send a British fleet into the Baltic and land forces to join with Russian land forces on the Pomeranian coast of Germany. At this time the USSR was not an ally, and the current British goals were therefore more modest.

The Admiralty plans division gave an immediate response to a 6 September query from Churchill on a possible Baltic offensive, stating that the operation justified detailed planning, but that Italy and Japan (the UK’s two navally most threatening possible opponents) must still be neutral for the launch of any operation in the Baltic, where the threat of air attack to British warships appeared a very significant obstacle.

Churchill hoped that the presence of a British fleet in the Baltic could dominate this sea, cutting off the flow of iron ore from Sweden and generally isolating Germany from all of its Scandinavian trade. It would also have the advantage of securing the Finnish, Swedish and Norwegian trade for the UK and interdicting German seaborne trade with the USSR.

Cork started work immediately and presented his preliminary appreciation on 26 September. Cork and Churchill agreed that the operation would require battleships with special modifications to cope with concentrated air and U-boat attack. Since there was inadequate time for such ships to be built, it was proposed to modify two or three of the Royal Navy’s oldest capital ships, battleships of the 'Revenge' class: super-bulges along their water lines would improve survivability in the event of torpedo attack, while extra armour plating on their decks would provide enhanced protection against aerial bombs.

The earliest time at which British dockyards could complete this work was late in the spring of 1940. Cork’s preliminary study described the operation as hazardous but feasible. It was assumed that Cork would be the fleet commander, and he asked for a margin of at least 30% over the German fleet on account of expected losses in the passage through the narrow seas off Denmark.

Fuel for three months of operations was to be allocated, and the fleet was also to include three heavy cruisers with 8-in (203-mm) guns and two light cruisers with 6-in (152-mm) guns, two flotillas of the newest destroyers, a detachment of submarines, and the relevant repair and depot ships.

It was hoped that the presence of a powerful British fleet dominating the Baltic for three months would prompt Sweden to offer the British a naval base. Failing that, the fleet would withdraw before exhausting its fuel supplies.

Cork’s study also stated the absolute need to assemble his fleet and finish training by mid-February 1940. Since the modifications to the 'Revenge' class battleships could not be finished by that time, however, the project was cancelled. A growing appreciation of the threat posed to warships by German warplanes, most especially dive-bombers and torpedo bombers, had already cast doubt on the feasibility of reviving of the project, which was already conceptually old, and then events in Finland, Denmark and Norway (Cork being directly involved in the last) between February and May 1940 rendered the plan obsolete.

Cork was later designated as the force commander of a planned Anglo-French expedition to assist the Finns in the 'Talvisota' winter war they were waging against a Soviet attack launched on 30 November 1939, but Sweden refused permission for the force to transit its territory and the expedition was cancelled.