High-gain Free Electron Lasers and future Linear Colliders require development of modern electron linacs with high brightness beams. Conventional intercepting transverse electron beam diagnostics, e.g. based on Optical Transition Radiation (OTR), cannot tolerate such high power beams without remarkable mechanical damages on the diagnostics device. Optical Diffraction Radiation (ODR) is an excellent candidate for measurements of the transverse phase space parameters in a non-intercepting way. One of the main problems of this method is the low signal to noise ratio, mainly due to the unavoidable synchrotron radiation background. This problem can be overcome by using two slits on metallic foils, placed at a distance shorter than the radiation formation zone. In this case a nearly background-free ODR interference pattern is produced allowing the determination of the beam size and angular divergence. The accuracy on these parameters can be increased by exploiting both ODR polarization states, as well as different wavelengths. Here we report measurements of the ODR interference between two slits with different aperture sizes in a non-collinear geometry, carried out at FLASH (DESY, Germany). Our results demonstrate the unique potential of this technique to determine the beam parameters.