This thesis aims to examine the impact of the British Labour Party and the Sozialdemokratische Partei Deutschlands ('Social Democratic Party of Germany' or the SPD)’s engagement in European integration on their political transformation. Labour and the SPD, two of the most prominent centre-left parties in Europe, have always been in the spotlight. Research has offered multiple accounts to explain their change over time, but generally neglected one factor: European integration. In an attempt to fill this gap, this study asks the following question: To what extent did Labour and the SPD’s involvement with the European project affect their domestic political evolution and attitudes to the European Communities (EC)/European Union (EU) based on their own experiences? This complex effect is investigated on the basis of party politics, policy-making, and ideological dimensions for the period covering from the mid-1980s until the 2010s. Centring on their experiences, this study locates Labour and the SPD as active agents rather than passive recipients in their handling of the European challenge.

In line with the multiple-case study design, semi-structured qualitative interviews and archival research were employed to collect the data. 32 senior political elites from Labour and the SPD (16 participants each), alongside three distinguished professors, were interviewed. Moreover, several selected physical and digital archives on Labour and the SPD’s European vocations were covered. To analyse the collected data, the reflexive thematic analysis (RTA) method was used, drawing on Peter Mair’s Europeanisation, party change, and cartel party theories as the main theoretical framework.

Based on this analysis, this thesis demonstrates that Labour and the SPD’s involvement with European integration exerted a remarkable impact on their political transformation. These findings indicate that although this process yielded different results in terms of party politics, there emerged more or less similar results regarding policy-making and ideological dimensions for both parties. The significance of this study is that it designates European integration as a salient factor in terms of party change in the particular context of the two parties.