On the afternoon of 20 May, 1941, the Swedish seaplane cruiser Gotland spotted the German warships Bismark and Prinz Eugen in the Kattegat between Denmark and Sweden. This information was passed on to the British naval attaché in Stockhlom, who then passed it to the British Admiralty.
The Admiralty was not oblivious to Raeder’s plan to unite the the German surface fleet in the Atlantic. In fact it was thier worst case scenario verbatim. With Bismarck’s sighting, requests went immediately to bomber command to sink or disable the Scharnhorst and Gneisenau in Brest, and track the Bismarck. the next day, plans were put into action to recall every available warship in the Atlantic and Indian Oceans.