Long-distance time and frequency transfer methods based on optical fibre links have evolved rapidly in recent years, demonstrating excellent performance for frequency transfer and considerable promise for accurate time transfer. Optical fibre links, besides provisioning extreme data transfer rates, are attractive both for very high-performance applications such as optical clock comparisons, basic research or relativistic geodesy, but also for many industrial and societal applications, and they complement and offer an alternative to radio- and satellite-based methods. CLONETS-DS is a European Union-funded research and innovation action intended to facilitate the vision of a sustainable, pan-European optical fibre network for precise time and frequency reference dissemination, by bringing together expertise from national metrology institutes (NMI), academic groups and research infrastructures (RI), research and education networks at the national and European level (NREN, GÉANT) and innovative high-tech small and medium enterprises (SME). The proposed project aims to establish a pan-European time and frequency reference system as a European Research Infrastructure which serves the European science community. It is based on transmitting ultra precise time and frequency information via optical fiber. The proposed project builds on several projects of the European metrology research programmes such as NEAT-FT, OFTEN and TiFOON, the former GN3+ project ICOF, the novel work package 6, Optical Time and Frequency Network (OTFN), of the GN4-3 project of GÉANT and its direct precursor project CLONETS . We now go far beyond previous efforts by designing a sustainable, pan-European, ultra-precise time-and-frequency reference-system available to the European research community. This newly designed European Research Infrastructure will consider user needs, architecture requirements, engineering models and roadmaps, and will develop a sustainability model for the future service, thus strengthening the European research area and its innovative potential.