This paper examines the evolution of British trade union policy relating to the European Union (EU). It focuses upon the 1975 UK referendum on continued membership of the Common Market (latterly the EU), and uses this key event to illuminate the range of the debate within the trade union movement, the rationale why it determined to oppose British membership of the EU and why its scampaigning proved largely ineffective, before considering the consequences arising from the referendum defeat. The paper identifies a number of issues resonant within the labour movement–including the decline in the strength of the left and the concomitant polarisation of opinion concerning the optimality of pursuing predominantly national or super-national economic and social policy–which have resulted in the periodic oscillation in trade union strategy, from opposition to (conditional) support for the European integration ‘project’. It surmises that the inability of the trade union leadership to construct a viable strategy, able to combine full employment with social and labour market protection for vulnerable workers, implies that the questions last comprehensively aired during the 1975 referendum campaign have never been satisfactorily resolved. Consequently, an understanding of the factors pertaining to the 1975 referendum campaign has the potential to inform the contemporary debate.