[en] We determined the neutron magnetic form factor G_mn via a measurement of the ratio of cross sections D(e, e'n) and D(e, e'p). The absolute neutron detection efficiency of the detector is measured with high precision using tagged neutrons produced by H(n, p)n scattering of a high intensity neutron beam. In a first experiment we have demonstrated that with this method G_mn can be determined with high accuracy (''+'' 1.7%). (author)