The cavity Beam Position Monitor (BPM) is a fundamental beam diagnostic instrument for a seeded FEL, like FERMI@Elettra. It allows the measurements of the electron beam trajectory in a non-destructive way and with sub-micron resolution. The high resolution cavity BPM is providing relies on the excitation of the dipole mode that is originated when the bunch passes off axis in the cavity. In this paper we present the prototype of cavity BPM developed for the FERMI@Elettra facility. The RF parameters of the cavities have been determined by means of Ansoft HFSS while using the CST Particle Studio the level of the output signals from the cavities have been also estimated. Furthermore, the design of the prototype electronics for the acquisition and g of the signals from the BPM cavities is presented as well. The prototype has been installed in the FERMI Linac during the last commissioning phase and preliminary results with the electron beam are also presented.