SARS-CoV-2 lineage B.1.1.7, a variant first detected in the UK in September 20201, has spread to several countries worldwide. Several studies have determined that B.1.1.7 is more transmissible than existing variants, but have not identified whether it leads to any change in the severity of the disease.2. Here we analyze a dataset that connects 2 245 263 positive SARS-CoV-2 community tests and 17 452 COVID-19 deaths in England from 1 September 2020 to 14 February 2021. For 1 146 534 (51%) of these tests, the presence or absence of B.1.1 .7 can be identified due to mutations in this lineage that prevent PCR amplification of the spike gene target1). Based on 4,945 deaths with a known SGTF status, we estimate that the risk of death associated with SGTF is 55% (95% AI 39-72%) higher after adjustment for age, gender, ethnicity, deprivation, care facility, local authority of stay and test date. This corresponds to the absolute risk of death for a 55-69 year old man who within 28 days after a positive test in the community of 0.6% to 0.9% (95% AI 0.8-1.0% ) has increased. If we correct for incorrect classification of SGTF and lack in SGTF status, we estimate a 61% (42-82%) higher mortality rate associated with B.1.1.7. Our analysis suggests that B.1.1.7 is not only more transmissible than existing SARS-CoV-2 variants, but that it can also cause serious illness.