A systematic comparison between the line integrated electron density derived from interferometry and polarimetry at JET has been carried out. For the first time the reliability of the measurements of the Cotton-Mouton effect has been analysed for a wide range of main plasma parameters and the possibility to evaluate the electron density directly from polarimetric data has been studied. The purpose of this work is to recover the interferometric data with the density derived from measured Cotton-Mouton effect, when fringe jump phenomena occur. The results show that the difference between the line integrated electron density from interferometry and polarimetry is with one fringe (1.143¥1019 m-2) for more than 90% of the cases. It is possible to consider polarimetry as a satisfactory alternative method to interferometry to measure the electron density and it could be used to recover interferometric signal when a fringe jumps occurs, preventing difficulties for the real-time control of many experiments at the JET machine.