With the Treaty of Lisbon, national Parliaments obtained a direct role in the legislative process of the European Union (EU). When the Commission releases a legislative proposal, each national Parliament has eight weeks to issue a Reasoned Opinion stating that the draft violates the EU principle of subsidiarity if they wish to do so. This article provides context on this so‐called Early Warning System (EWS), and then studies empirically when national Parliaments issue Reasoned Opinions under the EWS. A within‐between panel regression covering all 28 EU countries for 2010–16 leads to novel findings on the issuance of Reasoned Opinions. In particular, there is no robust statistical evidence that variations in public attitudinal euroscepticism in a country affect the number of Reasoned Opinions issued by its Parliament. In contrast, electoral euroscepticism as measured by the election of eurosceptic Parliaments is found to have a strongly significant across‐country effect on the number of Reasoned Opinions.