In early 1943, No. 62 Commando was disbanded and its members dispersed amongst other formations. Lassen was among a number who went to the Middle East to serve in the Special Boat Section, then attached to the Special Air Service. Others joined the 2nd SAS under the command of Bill Stirling, elder brother of David Stirling. During his time in the SBS, Lassen rose in rank to become a Major by October 1944. During his service he fought in North-West Europe, North Africa, Crete, the Aegean islands, mainland Greece, Yugoslavia and Italy. He was awarded two further bars to his Military Cross on 27 September 1943 and 15 February 1944.