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 (>100K) foreground synchrotron emission in the Milky Way and extragalactic continuum sources. 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 200MHz, corresponding to the redshift range 6-<z<13 for the 21-centimetre line. The data exclude a rapid reionization timescale of δz<0.06 at the 95% confidence level.
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