An employer can take into account prior misconduct for which a disciplinary warning has expired when deciding whether to dismiss for similar misconduct, if that misconduct in itself warrants dismissal. In contrast, if the misconduct is insufficient on its own to justify dismissal, dismissal based partly on the spent warning will be unfair (as held in Diosynth v Thomson).

The employer's procedure provided that warnings expired after 12 months and thereafter would normally be disregarded, although they could be borne in mind in deciding the level of disciplinary action where there was a pattern of abuse. The employee was found watching television during company time, held to be gross misconduct. In deciding to dismiss rather than give a final written warning, the employer took into account that the employee had a final written warning for gross misconduct, also related to misuse of company time, which had expired three weeks earlier. Other employees were given final written warnings for the same misconduct due to their prior good disciplinary records. The Court of Appeal decided that, given that it was reasonable to dismiss for this instance of gross misconduct, the fact than an employee had done this sort of thing before was a relevant circumstance in deciding whether or not to reduce the penalty. (Airbus UK Limited v Webb, CA)

Employers should treat this decision with caution. It may not be reasonable to use prior misconduct for which a warning has expired in this way in different circumstances, eg where the prior misconduct was of a different nature or, perhaps, took place much longer ago. In any event dismissal must be justified had there been no previous misconduct.

The Acas Code of Practice recommends that warnings should normally be subject to expiry after a certain period, such as 12 months for a final written warning. The Court here noted that employers can extend this period where the nature of the misconduct justifies it, for example if the warning is given instead of dismissal as an act of leniency. A warning could also be expressed as having a longer period of currency in relation to any later act of gross misconduct which is the same or substantially the same as that for which the earlier final warning was given. Employers' disciplinary policies should ideally set out the right to impose a longer period (or, alternatively, to refer to the reason for an expired warning) in these circumstances. (If an employer wishes to use historic disciplinary records, including expired warnings, for redundancy selection, this should also be expressly stated).