  The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised
by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with
numerous models suggested for the events. Here, we report multi-wavelength
observations of EP240315a, a 1600 s long transient detected by the Einstein
Probe, showing it to have a redshift of z=4.859. We measure a low column
density of neutral hydrogen, indicating that the event is embedded in a
low-density environment, further supported by direct detection of leaking
ionising Lyman-continuum. The observed properties are consistent with EP240315a
being a long-duration gamma-ray burst, and these observations support an
interpretation in which a significant fraction of the FXT population are
lower-luminosity examples of similar events. Such transients are detectable at
high redshifts by the Einstein Probe and, in the (near) future, out to even
larger distances by SVOM, THESEUS, and Athena, providing samples of events into
the epoch of reionisation.
