A method of automatically classifying alerts issued by intrusion detection sensors (11a, 11b, 11c) of an information security system (1) for producing collated alerts, each alert being defined by a plurality of qualitative attributes (a.sub.1, . . . , a.sub.n) belonging to a plurality of attribute domains (A1, . . . , An). Attributes belonging to each attribute domain are organized into a hierarchical structure. For each alert issued by the intrusion detection sensors (11a, 11b, 11c), a trellis specific to that alert is constructed by generalizing each alert in accordance with each of its attributes and at all the levels of the hierarchical structure. Each specific trellis is iteratively merged into a general trellis. Collated alerts in the general trellis are identified by selecting the alerts that are simultaneously the most pertinent and the most general. The collated alerts are supplied to an output unit (23) of an alert management system (13).