To what extent does political practice under the British Conservative-Liberal Democrat coalition (2010–2015) reflect a ‘parliamentary prerogative’? From a formal-institutional point of view one should not expect substantial parliamentary influence in Britain. Yet recent developments suggest the emergence of a new convention. Examining parliamentary debates during the run-up to the votes on Libya and Syria, this contribution shows that the scope and contents of this convention remain contested. Specifically, there is disagreement about the kind of operations that ought to be exempt from the rule, questions of parliamentary procedure that favour the executive and, crucially, the proper timing of substantive votes. Nonetheless, parliament has emerged from the vote on Syria as an informal veto player on decisions regarding war involvement. However, whether MPs will exercise their veto power in prospective cases will depend on the preference distribution in the legislature and the nature of the proposed deployment.