  Globular clusters are some of the oldest bound stellar structures observed in
the Universe. They are ubiquitous in large galaxies and are believed to trace
intense star formation events and the hierarchical build-up of structure.
Observations of globular clusters in the Milky Way, and a wide variety of other
galaxies, have found evidence for a `metallicity floor', whereby no globular
clusters are found with chemical (`metal') abundances below approximately 0.3
to 0.4 per cent of that of the Sun. The existence of this metallicity floor may
reflect a minimum mass and a maximum redshift for surviving globular clusters
to form, both critical components for understanding the build-up of mass in the
universe. Here we report measurements from the Southern Stellar Streams
Spectroscopic Survey of the spatially thin, dynamically cold Phoenix stellar
stream in the halo of the Milky Way. The properties of the Phoenix stream are
consistent with it being the tidally disrupted remains of a globular cluster.
However, its metal abundance ([Fe/H] = -2.7) is substantially below that of the
empirical metallicity floor. The Phoenix stream thus represents the debris of
the most metal-poor globular cluster discovered so far, and its progenitor is
distinct from the present-day globular cluster population in the local
Universe. Its existence implies that globular clusters below the metallicity
floor have probably existed, but were destroyed during Galactic evolution.
