  We present optical, X-ray and gamma-ray observations of GRB 111209A, at a
redshift of z = 0.677. We show that this event was active in its prompt phase
for about 25000 seconds, making it the longest burst ever observed. This rare
event could have been detected up to z ~ 1.4. Compared to other long GRBs, GRB
111209A is a clear outlier in the energy-fluence and duration plane. The
high-energy prompt emission shows no sign of a strong black body component, as
expected if the event was caused by a tidal disruption event or a supernova
shock breakout. Given the extreme longevity of this event, and a lack of a
supernova signature, we propose that GRB 111209A is a relatively rare stellar
collapse of a low metallicity blue super giant star. Only this progenitor can
supply mass to the central engine over a duration of thousands of seconds.
Hence, GRB 111209A could have more in common with population III stellar
explosions, rather than normal long gamma ray bursts.
