  It is now generally believed that galaxies were built up through
gravitational amplification of primordial fluctuations and the subsequent
merging of smaller precursor structures. The stars of the structures that
assembled to form the Milky Way should now make up much or all of its bulge and
halo, in which case one hopes to find "fossil" evidence for those precursor
structures in the present distribution of halo stars. Confirmation that this
process is continuing came with the discovery of the Sagittarius dwarf galaxy,
which is being disrupted by the Milky Way, but direct evidence that this
process provided the bulk of the Milky Way's population of old stars has so far
been lacking. Here we show that about ten per cent of the metal--poor stars in
the halo of the Milky Way, outside the radius of the Sun's orbit, come from a
single coherent structure that was disrupted during or soon after the Galaxy's
formation. This object had a highly inclined orbit about the Milky Way at a
maximum distance of $\sim$ 16 kpc, and it probably resembled the Fornax and
Sagittarius dwarf spheroidal galaxies.
