I believe the event you're referring to is Operation Catapult, a British naval operation undertaken in early July 1940, once it had become clear that the French had collapsed and were unwilling to provide further resistance. British forces consequently impounded French ships in British ports and ordered French naval forces to sail from their bases to British ports so that they would not fall into Axis hands. When the latter was refused, they were instead ordered to sink French shipping to deny it to the Axis, including the now-infamous attack on Mers El-Kebir, which saw the sinking of a number of French ships and the loss of several thousand lives.

There was a somewhat mixed response to Operation Catapult in global terms, as to many countries, whilst somewhat ruthless, it helped to underline Britain's will to resist, although the Axis did also receive significant ammunition for propaganda use from the operation. Perhaps most importantly however, Catapult poisoned the well of relations between the French and the British, a relationship further strained by other transgressions, and encouraged particular hatred from the French Navy. There were comparatively few direct repercussions however; although the French Navy grew to loathe Britain (although it had been Anglophobic even prior to the war), they rarely ventured out of port, and the only instance that it directly backfired on Britain was by encouraging staunch resistance during Operation Torch in November 1942, and several running naval battles ensued off the coast of North Africa.