Executive autonomy influences the ability of states to make credible commitments, utilise their domestic resources, and deter adversaries. In majoritarian parliamentary regimes, it is often assumed that executive autonomy is derived from the possession of a substantial parliamentary majority, since this affords the government a ‘buffer’ in the legislature. Yet, this understanding fails to account for the value of seats above the majority threshold for foreign policy, where the executive is constrained by internal dissent more than the non-passage of legislation. This article challenges the assumption of a linear relationship between seat share and executive autonomy through a theoretical re-examination of the politics of foreign policy in majoritarian parliamentary systems. It argues that possession of a significant majority can actually undermine the executive’s foreign policy autonomy, since it increases the degree of intra-party factionalism without introducing corresponding benefits in the legislature. The argument is illustrated empirically through an examination of the politics of British foreign policy at three decisive elections (1950, 1955, and 1966) in the early Cold War period.