Once a disciplinary warning lapses, can it be relied upon in later disciplinary proceedings?

The ACAS Code of Conduct states that disciplinary warnings should be disregarded after a fixed period of time, normally not more than one year.

In Diosynth Ltd v Thomson [2006] Mr Thompson was given a written warning for a safety violation, which was to remain on his record for 12 months. After the written warning had expired, there was a fatal explosion at the plant and it emerged that Mr Thompson was one of the people responsible. His employer dismissed him, taking into account the previous warning as they thought him unlikely to improve despite the training he had received.

Mr Thompson claimed unfair dismissal and the (Scottish) Court of Session agreed with him. The expiry provision in the ACAS Code of Practice is there for a good reason; an expired warning should not hang over an employee indefinitely. In this particular instance the employers had said that they would not have dismissed Mr Thompson if he had not received the earlier warning. His dismissal was therefore unfair.