This paper utilises the 1975 UK referendum on continued membership of the Common Market (latterly the European Union) to examine the evolution of British trade union policy relating to the European Union. The referendum generated debate surrounding issues of national economic management, political sovereignty and the optimum way to pursue progressive employment and social policy. The lack of focus upon the referendum campaign by the trade union movement, and the resultant ineffective campaigning, is explained by a combination of perceived prioritisation of national policy priorities and internal division on the issue. Many of the issues raised have never been satisfactorily resolved, resulting in subsequent periodic oscillation in trade union strategy. Thus, an understanding of the factors pertaining to the referendum campaign has the potential to inform the contemporary debate.