It is clear that a strand of anti-Stalinism had firmly entered the political discourse of British Labour and the Left by the late 1940s. This was largely a response to mounting Cold War tension and is rightly seen to contrast with their earlier broad support for the Soviet experiment. This article adds to this picture by arguing that anti-Stalinism was first adumbrated in the late 1930s in reaction to the Stalinist purges. Whereas previous accounts have given the impression that the Labour party and the wider Left were largely uninterested in the purges, this article shows instead that they provoked sharp criticisms of Stalin from across the range of Labour and Left opinion. Moreover, it also demonstrates that while certain figures and groupings did choose not to discuss the purges publicly, this was not necessarily because of a lack of interest or knowledge. For the Left the domestic political necessities of supporting the united front meant that, at least temporarily, their real views had to be concealed. More generally, the international pressure of endorsing the Soviet Union as an ally against international fascism inhibited an even more forthright condemnation of Stalin.