The sustainability performance of five potential UK biodiesel feedstocks is reviewed and their greenhouse gas performance investigated using the carbon and sustainability reporting methods of the UK Renewable Transport Fuel Obligation (RTFO). Of the feedstocks examined, and for which the RTFO supplies default carbon intensity values, only used cooking oil has guaranteed sustainability benefits. The other feedstocks have CO2e payback periods of 25–5503 years if grown on converted forest or grassland, yet the RTFO currently requires no guarantee that this conversion has not taken place and requires no avoidance of indirect effects. As currently designed, the RTFO risks substantial, adverse environmental effects.