The man, who has not been identified, forwarded a consultative memo from M&S to its 70,000 staff to the media in August. The memo described how the supermarket planned to reduced redundancy pay-outs by around 25%.
GMB's legal officer, Maria Ludkin, who represented the union's members at the disciplinary hearing, described the sacking as "a gross act of corporate bullying". The man left with £1,200, his outstanding holiday pay.
Ludkin said GMB will be appealing the decision and will launch a public campaign to secure justice for this M&S worker inside and outside the company.
"The disappointing part of today's decision is that M&S head of global HR John Wareham stated that the long service of this employee is totally irrelevant to the decision to sack him."
M&S internal investigators traced the employee, who had sent details of the plan to a newspaper, by analysing e-mail traffic. The firm suspended the worker, who has been with M&S for 25 years, on 21 August pending a disciplinary hearing and possible sacking.
M&S confirmed the sacking. In a statement it said, “We never take a decision like this lightly.”
The company took the decision because the man deliberately leaked internal information and made derogatory and speculative comments to the media, the statement said. This was despite other internal routes available to address his concerns.
“We could not be confident that the individual would not disclose internal company information in this way in the future,” it said.