Multi‐sector partnership working has become an increasingly important mode of governance across many Western European countries. It is seen as a means of overcoming social divisions, promoting more inclusive policymaking, and transforming governance systems. Partnership is perceived to be a more flexible form of governance, capable of resolving some of the complex policy and legitimation problems associated with more traditional statist approaches, and thus preferable for the delivery of public policy and services. However, as previous research has shown, there is an emerging ‘partnership crisis’ with partnerships often failing to be sufficiently inclusive of representative interests, leading to a lack of legitimacy, equity and effectiveness. This article explores the unique approach to addressing the lack of balanced and effective representation in partnerships developed by the Welsh Assembly Government. The Assembly has adopted a formal approach to structuring key partnerships on the basis of strict equality of representation across the public, private and voluntary sectors — the so‐called ‘three‐thirds principle’. This approach is conceptualized as a form of metagovernance whereby formal influence over partnership structures is being used in an effort to create institutional spaces for inclusion. The analysis indicates that while such network design does achieve improvement in partnership legitimacy, on its own it does not increase partnership effectiveness which remains constrained by the prevailing emphasis upon narrow, managerialist implementation agendas.