'MAT1' was a British naval minelaying operation by Admiral Sir Andrew Cunningham’s Mediterranean Fleet in the Ionian Sea between Lefkada and Kefaloniá islands (21 May 1941).

The minefield was laid by the cruiser minelayer Abdiel, which was covered by the battleships Valiant and Warspite, the light cruiser Ajax, and a number of destroyers. Designed to catch Axis shipping using the Corinth Canal, the minefield sank its first Axis transport ships and escort vessels later in the same day.