Malta needed more forces

After the German capture of Crete with airborne forces, the British became increasingly concerned that the Germans might attempt the same sort of attack on Malta. The decision was made, perhaps in July 1941, that more troops and artillery needed to be shipped to Malta. With the Germans more active in the eastern Mediterranean Sea, the convoy would be sent from west to east. The force to be sent to Malta would include "two battalions of infantry, one heavy and one light anti-aircraft regiment, thirty field guns and then men to man them, and a number of Royal Air Force pilots and technicians". The operation ran from July 21 to 24, 1941, and was called Operation Substance. A considerable naval force was involved:

battleship Nelson
battlecruiser Renown
cruiser Edinburgh
cruiser Manchester
cruiser Arethusa
aircraft carrier Ark Royal
some destroyers
8 submarines

This is based on the account in Vol.II of the Official History. I also consulted this web page about convoys.