'MB5' was a British naval undertaking by Admiral Sir Andrew Cunningham’s Mediterranean Fleet to reinforce the garrison of Malta and also to attempt to entice the Italian fleet into a major surface action (26 September/3 October 1940).

The light cruisers Gloucester and Liverpool were loaded with 1,200 troops and essential supplies for the island, and departed Alexandria with cover provided by the battleships Valiant and Warspite, the fleet carrier Illustrious, the heavy cruiser York, the light cruisers Orion and Australian Sydney, and the destroyers Hereward, Hero, Hyperion, Ilex, Imperial, Janus, Jervis, Juno, Mohawk, Nubian and Australian Stuart of the 2^nd and 14^th Destroyer Flotillas.

Italian air reconnaissance discovered the movement of the British ships, and the Italian navy despatched from Taranto and Messina the modern battleships Littorio and Vittorio Veneto (9a Divisione Navi di Battaglia), modernised battleships Conte di Cavour and Giulio Cesare (5a Divisione Navi di Battaglia), heavy cruisers Pola, Zara, Fiume and Gorizia (1a Divisione Incrociatori), heavy cruisers Bolzano, Trento and Trieste (3a Divisione Incrociatori), light cruisers Luigi di Savoia Duca degli Abruzzi and Giuseppe Garibaldi (8a Divisione Incrociatori), light cruisers Eugenio di Savoia and Emanuele Filiberto Duca d’Aosta (7a Divisione Incrociatori) and 23 destroyers.

During the night of 28/29 October the British ships came under attack by 28 Savoia-Marchetti SM.79 torpedo bombers, which achieved no success, but were unable to respond as the Illustrious's nine available Fairey Swordfish torpedo bombers would have suffered very high losses in the face of local Italian air superiority and the anti-aircraft fire of so many Italian warships. The Italian fleet refused to be drawn into combat, however, and returned to port on 30 October.

Despite its failure to bring the Italian navy to action, the British operation was successful in its primary transport task, Gloucester and Liverpool unloading their troops and supplies in Malta on 30 October before returning to sea to link with the other British ships and head back to Alexandria.