[en] Using the L3 detector at LEP the cross section of single photon events has been measured in e⁺e^- collisions near the Z resonance. An integrated luminosity of 9.6 pb^-1 has been collected observing 202 single photons candidates with energy between 0.9 and 3.5 GeV. Assuming that the only stable, weakly interacting particles are light neutrinos with standard model couplings, the number of light neutrino species has been determined to be N_ν=3.14±0.24(stat)±0.08(sys). This corresponds to an invisible decay width of the Z into weakly interacting particles of Γ_inv=524±40±13 MeV. (author) 11 refs.; 6 figs.; 1 tab