1. X-ray Astronomy Group, Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
2. Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking RH5 6NT, UK
3. XMM-Newton SOC, Villafranca, 28080, Madrid, Spain

Correspondence to: J. N. Reeves¹ Correspondence and requests for materials should be addressed to J.N.R. (e-mail: Email: jnr@star.le.ac.uk).

Now that -ray bursts (GRBs) have been determined to lie at cosmological distances¹, their isotropic burst energies are estimated to be as high as 10⁵⁴ erg (ref. 2), making them the most energetic phenomena in the Universe. The nature of the progenitors responsible for the bursts remains, however, elusive. The favoured models range from the merger of two neutron stars in a binary system^{3, 4, 5} to the collapse of a massive star^{6, 7, 8}. Spectroscopic studies of the afterglow emission could reveal details of the environment of the burst, by indicating the elements present, the speed of the outflow and an estimate of the temperature. Here we report an X-ray spectrum of the afterglow of GRB011211, which shows emission lines of magnesium, silicon, sulphur, argon, calcium and possibly nickel, arising in metal-enriched material with an outflow velocity of the order of one-tenth the speed of light. These observations strongly favour models³⁰ where a supernova explosion from a massive stellar progenitor precedes the burst event and is responsible for the outflowing matter.