  The discovery of a number of gamma-ray bursts with duration exceeding 1,000
seconds, in particular the exceptional case of GRB 111209A with a duration of
about 25,000 seconds, has opened the question on whether these bursts form a
new class of sources, the so called {\em ultra-long} GRBs, or if they are
rather the tail of the distribution of the standard long GRB duration. In this
Letter, using the long GRB sample detected by {\em Swift}, we investigate on
the statistical properties of ultra-long GRBs and compare them with the overall
long burst population. We discuss also on the differences observed in their
spectral properties. We find that ultra-long GRBs are statistically different
from the standard long GRBs with typical burst duration less than 100-500
seconds, for which a Wolf Rayet star progenitor is usually invoked. We
interpret this result as an indication that an alternative scenario has to be
found in order to explain the ultra-long GRB extreme energetics, as well as the
mass reservoir and its size that can feed the central engine for such a long
time.
