'MB9' was a British naval undertaking within 'Collar' (ii) by Admiral Sir Andrew Cunningham’s Mediterranean Fleet to secure the safe passage of the MW.4 convoy (23/28 November 1940).

The MW.4 convoy comprised four transports (7,347-ton Clan Ferguson, 10,492-ton Clan Macaulay, commissioned 10,000-ton Breconshire and 7,506-ton Memnon) routed from Alexandria to Malta round the northern side of the island of Crete. The convoy was escorted by the battleship Ramillies and heavy cruiser Berwick which, after detaching the merchant ships to Malta, continue to Gibraltar after being supplemented by the light cruiser Newcastle, already in Malta where, on 19 November, it had delivered RAF personnel and matériel.

Of the various naval forces and convoys involved in this complex operation, the first to sail was Force 'D', which included the light anti-aircraft cruisers Calcutta and Coventry along with the destroyers Greyhound and Australian Vampire, Vendetta and Voyager. Force 'D' was tasked with the escort of the MW.4 convoy, which departed Alexandria on 23 November and passed round the north of Crete. The convoy had the more distant support of Force 'C', which included the 1^st Battle Squadron with the battleships Malaya and Ramillies, the old carrier Eagle only recently brought back into service after repairs had been effected, the heavy cruiser Berwick, the light cruisers Ajax, Orion and Australian Sydney (Rear Admiral H. D. Pridham-Wippell’s 7^th Cruiser Division) and the destroyers Dainty, Diamond, Gallant, Hasty, Havock, Hyperion and Ilex.

A second battle group, Force 'A' including the fleet carrier Illustrious, the battleships Warspite and Valiant, and the destroyers Decoy (for repairs at Souda Bay), Griffin, Janus, Jervis, Juno, Mohawk, Nubian, Wryneck and Australian Waterhen, sailed on 25 November to provide further support. Force 'A' also passed to the north of the island of Crete, pausing to fuel at Souda Bay, and on 26 November was already on its way to Malta.

At about 12.00 on 27 November, this naval force was only some 185 miles (300 km) from the island. A separate attack was conducted by the Fairey Swordfish bombers of Eagle against the Italian port of Tripoli in Libya. The returning crews reported the sinking of one ship, but in fact the Italians suffered no losses. Yet another element of the operation, Force 'F' including the 3^rd Cruiser Division with the heavy cruiser York and the light cruisers Glasgow and Gloucester, was already at sea on a training exercise, and was later merged with Force 'A'.

Early in the morning of 26 November, Illustrious, Glasgow, Gloucester and four destroyers separated from Force 'A' to launch and air raid against the Italian base of Portolago on the Aegean island of Léros in the Dodecanese islands group. The mission was partially successful, but No. 819 Squadron lost one of its Swordfish aircraft.