  A non-uniqueness problem of gauge invariant separation of quark and gluon
contributions to nucleon spin is considered. We show that there is a wide
number of gauge invariant spin decompositions each of them reduces to the
canonical one in a special gauge. A class of physical gauge equivalent nucleon
spin decompositions is selected by requirements of consistence with helicity
notion described within E(2) little group representation theory and with gluon
helicity \Delta g measured in experiment.
