  We report the results from a study of ~23 Million K+- ==> pi+- pizero pizero
decays recorded by the NA48/2 experiment at the CERN SPS, showing an anomaly in
the pizero pizero invariant mass distribution in the region around 2m+, where
m+ is the charged pion mass. This anomaly, never observed in previous
experiments, can be interpreted as an effect due mainly to the final state
charge exchange scattering process pi+ pi- ==> pizero pizero in K+- ==> pi+-
pi+ pi- decay. It provides a precise determination of a0 - a2, the difference
between the pi-pi scattering lengths in the isospin I=0 and I=2 states.
