System for starting authorization for a motor vehicle

The invention relates to a hands-free vehicle engine start authorization system comprising an onboard user identification device (1) capable of exchanging identification data via a transmission antenna (7) according to an "active" mode of communication with an identification unit carried by a user. The onboard identification device (1) further comprises electronic adaptation means (9), (6), (10) of the transmission antenna to enable a "passive" mode of communication as a backup solution of the device (1) with a transponder (3).