British code-breakers of the Government Code and Cypher School (GC&CS) at Bletchley Park in England, deciphered Italian orders of 19 May, coded using C38m machines, secretly to mobilise the army and air force in East Africa. Merchant traffic was stopped by the British on 24 May, pending the introduction of a convoy system. The Red Sea Force (Senior Naval Officer Red Sea, Rear-Admiral Murray), operational at Aden since April with the light cruisers and HMAS "Hobart" ("Liverpool" was replaced by ), was reinforced by the anti-aircraft cruiser , which sailed south with Convoy BS 4, the 28th Destroyer Flotilla comprising , "Kimberley", and and three sloops from the Mediterranean. The force was to conduct a blockade Italian East Africa (Operation Begum), attack the Red Sea Flotilla and protect the sea lanes from Aden to Suez.