The radial electric field gradient or the E x B flow shear has been pointed out as the underlying mechanism for turbulence suppression, responsible for an internal transport barrier formation in advanced tokamak scenarios. A comprehensive study on these subjects requires a direct measurement of the plasma radial electric field Er. The poloidal component of the magnetic field is assessed by the Motional Stark Effect (MSE) polarimeter, which is currently a standard diagnostic in fusion devices, allowing a local and non perturbative measurement of the magnetic pitch angle. A precise measure to the state of polarisation of the Stark components gives the information on the direction of the magnetic field. Due to the particular orientation of the Lorentz component, that is nearly perpendicular to Er, the MSE diagnostic is very sensitive to the plasma intrinsic radial electric field. This paper describes a technique to measure Er involving the change of the polarisation angle of the MSE emission, by using two beam injectors at different energies, firing sequentially. Experimental results for the low Er case, i.e. with very little plasma rotation, showing the ability of the MSE to perform this measurement, will be presented. This is the first time that evidence of a direct measurement of the plasma Er is reported from JET.