Oxo-degradable plastic packaging, including carrier bags, have in recent years been marketed as a solution to plastic pollution, with claims that such plastics, when they end up in land or aquatic environments, degrade into harmless residues within a period ranging from a few months to several years. However, a significant body of evidence indicates that oxo-degradable plastics simply fragment into small pieces, including microplastics, with the entire process of biodegradation into naturally occurring molecules requiring timescales often (far) in excess of those claimed by their manufacturers. The contribution of these plastics to microplastic pollution poses an environmental risk, particularly in the ocean. Furthermore, oxo-degradable plastics are not suited for effective long-term reuse, recycling at scale or composting. In summary, the evidence to date suggests oxo-degradable plastic packaging goes against two core principles of the circular economy.