The unnamed staff member was fired by M&S earlier this week following a disciplinary hearing, prompting criticism of the high street giant by union representatives.
The mole, who reportedly worked in the company’s IT department and had been on the payroll for 25 years, was initially suspended after M&S’ plans to cut its redundancy package appeared in the press.
The GMB union has indicated that it intends to protest against the retailer’s actions.
In a statement M&S said: “We're both surprised and disappointed by the GMB's actions. Despite claims they will appeal on behalf of the individual, the GMB appears intent on conducting the case through the media, seemingly to gain publicity for their own ends.”