On 3 April, the British began to receive reports of a heavy buildup of shipping and troops in the Baltic German ports of Rostock, Stettin and Swinemunde. It was assumed that it was part of a force being sent to counter an Allied move against Scandinavia (the Germans had some awareness of Allied plans as a result of their own intelligence) and so that day, the British took the decision to proceed with the mining of the iron ore route separately from Operation Royal Marine, setting a date of 8 April for the Admiralty to implement it.