A double retarding field analyser for use in axially inhomogeneous axisymmetric magnetic field configurations is described and applied for measuring anisotropic electron distribution functions in an ECR discharge. The analyser uses the velocity transformation due to the constancy of the magnetic moment at adiabatic motion in an axially varying magnetic field. Its principles of operation and construction are described and discussed. An application to measuring the distribution function of electrons escaping through the mirror throat from an ECR discharge confined by a magnetic mirror trap clearly indicates an anisotropy in the energy distribution. The kinetic temperature of the gyrational motion of the electrons exceeds that of the motion parallel to the field lines.