'Lyster' was a British pair of naval undertakings, otherwise known as 'EF' (i), to block the Belgian ports of Zeebrugge and Ostend before the arrival of German forces (25/26 May 1940).

The planned demolitions at Zeebrugge were the responsibility of the French, who encountered considerable opposition from the Belgian authorities. On 25 May the British then made the first attempt to block the port using a force, under Captain G. A. Garnons-Williams, which had sailed from Sheerness in the destroyer Vega with two blockships for Zeebrugge and three for Ostend. The Admiralty cancelled the blocking of Ostend shortly after the force had departed, probably because it was expected that more evacuations would be made from that port.

The blocking force was heavily bombed while on passage, but suffered no serious damage, but then, on entering Zeebrugge, came under French fire from soldiers who, having no knowledge that the British intended to block the port, presumed that the ships were German. The first blockship went off course and grounded, and the second scuttled herself ineffectively near the first.

The operation had therefore failed in its purpose, and the Admiralty at once decided to repeat it. On 26 May two of the blockships originally destined for Ostend, with Vega again in command, sailed from Sheerness for Zeebrugge and, on passage, came under attack by aircraft and S-boote, which inflicted no damage. In this second attempt, the object of the undertaking was successfully accomplished.

The Admiralty still desired to block Ostend, as no more evacuations were intended from this port, and sailed three more blockships for that purpose on 29 May. But after the Air Ministry had told the Admiralty that no air cover could be provided, the operation was cancelled. By this time the full endeavours of the Royal Navy and RAF were concentrated on the 'Dynamo' rescue of the British Expeditionary Force from Dunkirk, and no forces could be spared to complete the obstruction of the Belgian ports.