This article re-examines Anglo-Portuguese relations in the first half of George Canning's second foreign secretaryship. It contends that Canning's determination to maintain British influence in Portugal led him to interfere in its internal affairs much more deeply and constantly than hitherto believed, and that, in doing so, he was motivated by his belief that British predominance in Lisbon was an essential element in the European balance of power. It will be suggested that Harold Temperley's widely accepted portrayal of Canning as a committed isolationist and a firm believer in the principle of non-interference in another state's internal affairs needs serious reconsideration.