Product labelling schemes have become one of the most prominently used instruments to facilitate more Sustainable Consumption and Production. But with a plethora of labelling schemes having been implemented, many now accuse them of being confusing rather than facilitating. As a result, governments in France, UK and Germany, as well as businesses, such as Walmart, and non-governmental organisations, like WWF, have begun to consider seriously the implementation of some form of ‘Sustainability Meta Label’ that condenses existing product-labels and other communication measures into a more coherent overarching scheme. Yet so far, in depth studies on the potential institutionalisation of a Sustainability Meta Labelling Scheme are missing.Based on case study research of four existing product labelling schemes (EU eco, EU energy, Fairtrade and MSC label), this study addresses this gap by developing a novel theoretical framework to study the causalities behind product labelling institutionalisation processes. Combining theoretical arguments of constructivist institutionalism and institutional entrepreneurship with concepts of legitimacy from the governance and organisational studies literature, this framework establishes the institutionalisation of product labelling schemes as contingent on an interactive legitimacy construction between actors involved in the initiation and organisational structures of a labelling scheme and other actors within the production and consumption system. This construction tends to cluster around aspects of tradition, regulation, charisma, knowledge, consequences, and procedures.By concretizing this framework in the context of the studied cases, it is shown how legitimacy constructions are highly complex and how in particular procedural and consequential legitimacy can give rise to fundamental conflicts. The potentially large scope, focus and area of application of a Sustainability Meta Label with the need to find agreements in regard to the very contested notion of Sustainable Development, seems to make the task of managing such conflicts even more difficult. While the mobilisation of knowledge, traditional, regulatory and charismatic logics can circumvent some of these conflicts, they have also demonstrated to be anything but a silver bullet. In a sense, this study shows that the very issue that is claimed to drive the establishment of a Sustainability Meta Labelling Scheme-the different interpretations of the Sustainable Development concept through different product labels-might in fact pose one of the main challenges for its institutionalisation and effectiveness in facilitating more Sustainable Consumption and Production.With these findings this study makes important contributions not only to an increasingly prominent policy making discussion but also to the wider product labelling and new institutional literature. After further empirical testing, the developed theoretical framework could guide future research into the institutionalisation of product labelling schemes and potentially also other ordering mechanisms. While the focus of this study is on commonalities across product labelling schemes such further research could especially expand on how micro, meso, and macro level factors can shape institutionalisation processes in diverse ways.