1. Over the last century butterflies have undergone substantial changes in abundance and range in Great Britain and monitoring has improved markedly. These changes, together with a major revision of International Union for Conservation of Nature (IUCN) criteria, render previous Red List assessments outdated.
2. A new Red List assessment of all 62 resident and regularly breeding butterfly species in Britain was undertaken. The current IUCN criteria were applied for the first time to British butterflies using data from the Butterflies for the New Millennium recording scheme and the UK Butterfly Monitoring Scheme.
3. The state of knowledge and nature of the data available from these two schemes enabled assessment to be based upon two quantitative IUCN criteria: A2 (rate of population decline) and B2 (area of occupancy).
4. Twenty-three species (37% of the total) qualified as Regionally Extinct (RE) or threatened: 4 species were listed as RE and 19 as threatened (two Critically Endangered, eight Endangered and nine Vulnerable). A further 11 species were classified as Near Threatened. The remaining 28 species (45% of the total) were classified as Least Concern.
5. In comparison with previous assessments, we show that the number of species classified as threatened has increased. This is likely to be due to a combination of refined assessment criteria, improved data, and a real increase in the extinction threat as a result of human impacts.
6. The results confirm butterflies as a highly threatened group of insects in Great Britain and the Red List provides an important foundation to define conservation priorities.