This article applies a mixed-methods approach through semi-structured interviews and document analysis to provide a comprehensive account of administrative and behavioural adaptation within the UK Houses of Parliament (HoP) to the EU’s subsidiarity monitoring mechanism, the Early Warning System (EWS). The article also tests theoretical assumptions regarding the adaptation and use of the EWS on this basis, confirming that Eurosceptic MPs bolster the use of the EWS and finding that the HoP are an outlier among bicameral legislatures, as the lower chamber was the primary user of the EWS. Overall, results demonstrate that both the House of Commons and the House of Lords treated the EWS as an optional bolt-on when adapting to the mechanism. Furthermore, the EWS did not encourage the HoP to increase engagement with UK devolved legislatures, but the mechanism contributed to the mainstreaming of EU scrutiny in the case of the Welsh and Scottish legislatures.

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This article, for the first time, analyses vertical networking among parliamentary groups and elected politicians from the same Europarty in the EU. It explores how, concerned about its growing ideological diversity, political fragmentation and recent sovereigniste tendencies, the European People’s Party Group in the European Parliament has sought to exercise strategic leadership within the EU multilevel parliamentary field by systematizing its cooperation with younger national MPs. Drawing on a mixed-methods approach including document analysis, elite interviews, and a participant survey, the article traces the origins, motivations, and implementation since 2016 of the EPP Group’s unique Erasmus Programme for visits by such national MPs to Brussels. It shows that participation enhanced the MPs’ knowledge about the EPP Group, the EP, and the EU. It also created new contacts between them and the EPP Group and other EP actors, and it contributed somewhat to legitimising the EPP Group’s role for national politics. It remains to be seen, however, whether increased vertical parliamentary networking will be both sustainable, not ephemeral, and transnational, not national—long-term effects that could only be traced with the help of a longitudinal research design.