'EF' (i) was a British unsuccessful naval undertaking to block the port of Zeebrugge on the coast of Belgium as the German forces approached it (25 May 1940).

The demolition of the port facilities at Zeebrugge was the responsibility of the French forces in the area but, like that of the British forces ordered to destroy the port of Ostend, the demolition was opposed by the Belgian authorities.

On 25 May, therefore, the British made their first attempt to block the port, the destroyer Vega departing Sheerness with the sloops Mallard, Sheldrake and Widgeon (replaced by Shearwater soon after the ships had sailed), minesweepers Fitzroy, Saltash and Sutton, two blockships for Zeebrugge (1,822-ton Florentino with the operation’s commander, Captain G. A. Garnons-Williams, on board and the 1,499-ton Transea) and three for Ostend; also involved were the motor torpedo boats MTB-15 and MTB-16, which were to be used to recover the skeleton crews of the blockships after they had been scuttled, and the minesweeping trawlers Clythness and Lord Grey.

The Admiralty cancelled the attempt to block Ostend shortly after the force had sailed, probably because it was expected that further evacuations would be made from that port. The six British ships were heavily bombed while on passage, but suffered no serious damage. When the ships entered Zeebrugge, however, they came under fire from French soldiers who, not having been informed that the British planned to block the port, probably assumed that the ships were German.

The first 'EF' (i) blockship strayed from her course and ran aground, and the second scuttled herself ineffectively near the first.

The Admiralty immediately decided to repeat the undertaking in 'Lyster', and on 26 May two of the blockships originally destined for Ostend, with Vega again in command, departed Sheerness. The ships were attacked at sea by German warplanes and S-boote, but suffered no damage and the blockships were successfully sunk in Zeebrugge harbour.

The Admiralty still wished to block Ostend, where no further evacuations were now planned, and despatched three more blockships on 29 May. On learning that the Royal Air Force could provide no air cover, however, the Admiralty cancelled the undertaking. By this time the full British naval and air effort was concentrated on 'Dynamo' farther to the west, and no forces could be spared to complete the physical obstruction of the Belgian ports.