A method of protection of a near-field contactless communication system against malicious attacks. The method includes exchange of information between a reader and a contactless card of duration T, measured with respect to a starting instant t0 seen from the reader, decoding of this information by the card, sending by the card a return signal temporally set with respect to an instant t′0+T, where t′0 is the starting instant as seen by the card taking into account delays in propagation or processing of signals received from the reader, detection of the return signal by the reader, determination of the temporal setting of the return signal with respect to the starting instant t0, and interruption of communication by the reader if the temporal setting of the return signal detected is not equal to the instant t0+T with a predetermined margin of error. The return signal is a pseudo-random sequence.