Identification system for proving access right to an object or to use the object, especially a motor vehicle

System comprises a portable key or signaler that exchanges signals with an onboard transceiver. The interrogation signal (F) from the onboard unit and the response signal (A) are both divided into sections (Fi, Ai) with a section of the response signal only sent when a request section has been received. The time (TFAD2) from the beginning of transmission of the first section (F1) to the last response signal (AN) is measured and compared with a reference value. Vehicle access is only authorized when the response signal contains the required code information and is sent within a time (TFAD2) shorter than the reference time.