A depression running across the outer Milne Ice Shelf was hypothesized to overlie a basal channel incised by outflow from the Milne Fiord epishelf lake, a thick layer of freshwater impounded in the fiord by the ice shelf. Ice thickness mapping using ice-penetrating radar revealed the presence of a channel with incision heights of 39 to 45 m (70-80% of mean ice shelf thickness), basal widths of 57-86 m, and mean sidewall slopes of ~40° upward from horizontal. Profiles of salinity, temperature and current speed with depth showed there was a fast flowing jet of epishelf lake water in the channel, with velocities up to 60 cm s-1, confirming the channel is a drainage outlet for the epishelf lake. The presence of the channel represents a significant structural weakness along which future breakup of the ice shelf is likely to occur.