Some EU agencies have been recently entrusted with enforcement powers, which imply a crucial extension of their regulatory reach. However, other comparable agencies did not receive such powers. This paper explores the case of energy regulation as an instance of these “negative” cases, and suggests that the lack of enforcement powers may have been partially determined by business interest groups. To illustrate this argument, this article firstly relies on official documentation to show that key interest groups were consistently opposed to the option of granting enforcement powers to the EU agency in charge (ACER). Secondly, it is suggested that these interest groups, which have been largely incorporated in regulatory networks during the prehistory of the agency, had access to, and exerted influence in, the governance of EU energy policy, and could plausibly have been able to concretise their preferences. A systematic examination of the representation of interest groups in the European network of energy regulators (CEER/ERGEG) during the period 2004–2011 is undertaken to corroborate this point. The conclusion draws attention to the fact that, although interest groups are less visible than other actors and their presence is less formalised, they could be very influential on decision-making processes within European networks and agencies.