Bose-Einstein correlations are studied in pairs of charged pions from hadronic Z decays, collected by the ALEPH detector. The correlation function, measured using either the unlike-sign or the mixed reference sample, is studied in terms of the Lorentz-invariant four-momentum difference and its transverse, Q T, and longitudinal, Q L, components with respect to the longitudinal centre-of-mass system. Values for the correlation radii, R T and R L, are obtained from the fit of the Goldhaber parametrisation. The results indicate that the correlation radii values depend on the chosen kind of reference sample and on the two-jet purity.