In a diesel engine equipped with a system for automatic regeneration of the particulate filter, the automatic regeneration mode is inhibited in the case where there is recorded a number higher than a pre-set threshold of unfavourable events, i.e., of interruptions of the automatic-regeneration step before its completion, and/or in the case where an insufficient quality of the engine lubricating oil is detected due to a dilution in the oil of fuel injected with the delayed injections that are necessary for regenerating the filter. When the automatic regeneration mode is inhibited, an on-demand regeneration mode is simultaneously enabled, which can be activated manually by the driver. The system checks in this case that on-demand regeneration is performed before the vehicle has covered a certain mileage since the automatic regeneration mode was inhibited. Otherwise, the vehicle is set in a condition of limited performance, to induce the driver to execute the manual regeneration mode. If this is not carried out before a further mileage is covered, also manual regeneration is inhibited, rendering regeneration possible only in a repair shop.