  Observations of the 21-centimetre line of atomic hydrogen in the early
Universe directly probe the history of the reionization of the gas between
galaxies. The observations are challenging, though, because of the low expected
signal strength (~10 mK), and contamination by strong (>100 K) foreground
synchrotron emission in the Milky Way and extragalactic continuum sources2. If
reionization happened rapidly, there should be a characteristic signature
visible against the smooth foreground in an all-sky spectrum. Here we report an
all-sky spectrum between 100 and 200 MHz, corresponding to the redshift range 6
< z < 13 for the 21-centimetre line. The data exclude a rapid reionization
timescale of dz < 0.06 at the 95% confidence level.
