A seat belt warning system comprises a sensing system associated to a vehicle seat for sensing a seat belt related status with respect to said vehicle seat and a control unit communicating with said sensing system for generating a warning signal depending on the seat belt related status determined by said sensing system. The sensing system comprises a seat occupancy detector (16) for determining a seat occupancy status of a vehicle seat and a seat belt fastening detector (18) for determining a seat belt usage condition. According to the invention said sensing system further comprises a first surface acoustic wave device (26) including at least one surface acoustic wave resonator (26') and an antenna (26'') and in that said control unit (20) comprises an RF antenna (22) for remotely communicating with said surface acoustic wave device (26).