An off-duty police officer shouted and made rude hand gestures at rival fans at a Swansea-Cardiff football match, then told an inspector who was policing the game that he was a postman.
Jonathan O'Rourke was captured on camera at the Liberty Stadium on October 27 last year when Swansea City beat rivals Cardiff City 1-0.
The Dyfed-Powys Police constable was due to attend a police misconduct hearing on February 28, but he resigned four days earlier.
The hearing, which went ahead in his absence, said Mr O'Rourke was seen shouting and gesticulating at nearby Cardiff supporters after the match had ended.
When approached by a South Wales Police inspector, who had been advised he was a constable or police community support officer (PCSO), Mr O'Rourke said he was a postman.
He was then seen passing his wallet to another person - and later said he was a PSCO.
When the 26-year-old's true identity was discovered, the matter was referred to Dyfed-Powys Police's professional standards department.
Mr O'Rourke, who was born in Swansea but was serving as a constable in Newtown, Powys, said when interviewed on December 11 last year that he was ashamed and embarrassed at his actions.
He said he had panicked and wanted to hide the fact he was a police officer.
Mr O'Rourke went on to admit that his lies breached honesty and integrity standards, and that his behaviour towards the opposition fans breached discreditable conduct standards.
Dyfed-Powys Police Federation member, PC Nick Jenkins, who represented Mr O'Rourke at the hearing, said: "In this case, gross misconduct is admitted."
After adjourning, the force's Chief Constable, Mark Collins, said he had taken into account Mr O'Rourke's antecedent history.
Mr Collins said the officer's behaviour at the match and subsequent lies "are so serious that dismissal would have been warranted in order to maintain public confidence in the police service".
Mr O'Rourke was placed on the police barred list for five years.
He can appeal to the police appeals tribunal within 10 working days.