'Foil' was a British midget submarine attack on the underwater telephone and teleprinter lines in the Lamma Channel off Japanese-occupied Hong Kong (31 July 1945).

The two tasks earmarked by the British for their midget submarine force, against considerable US Navy opposition, were the destruction of Japanese cruisers anchored in shallow waters, and the cutting of the underwater telephone cables between Singapore, Saigon, Hong Kong and Tokyo. The purpose this latter was to force the Japanese to resort to radio communication, thereby making it easier to intercept and thus be in a position to decrypt their communications traffic.

Tasked with tackling the Hong Kong end of the cable, 'Foil' was entrusted to Lieutenant H. P. Westmacott’s crew in XE-5, which was to be towed from the US Navy base at Subic Bay in the Philippine islands by the British submarine Selene. As well as operating well inshore close to Lamma island, Westmacott’s divers (Lieutenant B. G. Clarke and Sub Lieutenant D. V. M. Jarvis) had a very hard time of it as they were impeded by the bottom’s thick mud under the constant threat of 'oxygen pete', a poisoning caused by the use of too much oxygen under pressure. Jarvis had made his sortie dressed only in swimming trunks and breathing apparatus, and had the misfortune to operate the cutter on his finger, breaking the bone and causing heavy bleeding. In addition he was badly stung by Portuguese man o’war jellyfish. Despite all the challenges, the operation was a success.