A senior manager at Friday-Ad was sacked after he was caught running his own business from the office.
Rob Alder, who earned £42,000 as head of marketing at the company based in Sayers Common, near Burgess Hill, lost his job after junior colleagues discovered he was running his own web site from his desk.
Mr Alder, who worked for the company for five years and is now unemployed, claimed compensation at a Brighton employment tribunal for unfair dismissal.
The tribunal concluded he was unfairly dismissed because the company had not followed its own disciplinary procedure when Mr Alder was sacked without warning for gross misconduct in April.
But the panel ruled he was not entitled to any compensation because he breached company rules by working on another business during the time he was employed by Friday-Ad.
Chairman Lawrence Guyer said: "We conclude he was the author of his own misfortune."
During the hearing, colleagues told the tribunal how they became suspicious of Mr Alder's behaviour when he began to spend less time supervising their work and started making an increasing number of calls on his mobile.
He also spent time surfing the internet and was seen repeatedly looking at a web page called Sales-Coach.net, which promoted the services of a sales and marketing company offering training for the advertising industry.
When the web site was checked, Mr Alder's name was listed as an executive and his Friday-Ad company mobile number appeared as a contact number.
Managing director Robert Paterson described Mr Alder's behaviour as "blatant dishonesty."
He told the hearing: "Mr Alder had set up and was running his own business from our company offices. He was devoting his time and attention to running his own business and using our equipment to do it. There was no alternative but to dismiss him."
Mr Alder told the hearing he was shocked and stunned when he was sacked. He said he was dealt with unfairly because he was not given time to explain his actions.
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