For how long should warnings for poor performance remain "live"?
The starting point in determining for how long a warning for poor performance should remain "live" is to look at the employer's procedure. According to the Acas code of practice on disciplinary and grievance procedures, employers should set out the period for which warnings will remain active. The non-statutory Acas guide: discipline and grievances at work, which accompanies the code, states that warnings should normally be live only for a set period, for example six months for a first written warning and 12 months for a final written warning. The guidance does not rule out the possibility of an unlimited warning, although it will not normally be appropriate to issue an open-ended warning in a poor performance situation. If further underperformance occurs during the currency of a warning, the employer is able to take the warning into account when deciding what penalty to impose on the next occasion.
In most cases, once a warning has expired, it should not be taken into account by the employer when it is deciding the outcome of future poor performance proceedings. However, in Airbus UK Ltd v Webb  IRLR 309 CA, the Court of Appeal held that, in the context of a misconduct dismissal, employers may take into account expired warnings where this is not the principal reason for a subsequent dismissal (ie where the circumstances would have justified dismissal anyway). If, for example, an employee has a current final written warning for poor performance, and performance has not improved since the employer issued the warning, the employer might be considering dismissal (as it would be entitled to do as the employee's performance has not improved) or extending the final written warning if there are mitigating circumstances. On the basis of Airbus, it may take into account the previous expired warning in deciding that the mitigating circumstances do not warrant the lesser penalty.