An effective climate labelling scheme requires a methodology, a database, and a label format that allows consistent comparison both within and across product categories. To this end, we analyse the EU product environmental footprint (PEF) methodology, the state of databases on climate footprints, the current knowledge on effective label design, and relevant EU regulation. Based on this analysis, we conclude that further preparation is required before a voluntary, horizontal climate labelling scheme can be established under Union law, across all product categories. Specific improvements are proposed to harmonise and simplify the PEF methodology. We also propose that a globally complete, consistent, and open background database is established and maintained, with an acceptable level of product detail. A label design is proposed that allows seamless cross-category comparison and consideration of the 'monetary rebound' effect, as well as easy communication of uncertainty. The development of a roadmap is also proposed. This should consider the broader context of environmental and sustainability labelling and the need to improve international product life cycle assessment standards and harmonise conflicting EU calculation rules.