Now the 33-year-old councillor could receive a red card from his own party if he makes another appearance in a United shirt at a Labour-run Worcester City Council meeting.
Mr Gregson went to a meeting on Tuesday, kitted out in a United shirt, a pair of rugby shorts, tennis socks and training shoes, to protest against a dressing down directed at scruffy councillors by Labour's mayor-elect Les Thomas.
Mr Thomas, 61, who becomes mayor next month, told councillors that some of them looked like they were heading for the beach and needed to smarten up. He has said he intends to seek legal advice to banish members from the chamber unless they wear a jacket and tie.
"I'm quite sure last night was a protest against my remarks and I'm very disappointed," said Mr Thomas, who was defending his Nunnery ward seat in yesterday's local elections.
"I think it's bad form. It just showed a lack of dignity for the council and respect for the position. He did smile at me and I smiled at him but I was very disappointed ... Everybody treated it as a joke when I brought this up, but the point I was making was that people in the city look up to us and expect us to dress to a standard. We are, after all, the city fathers.
"Back in the 60's the High Sheriff of Worcester was digging his garden in an old pair of grey flannels and a shirt when a member of the public asked him if he thought it becoming to dig the garden in such old clothes.
"If that person had been in the chamber last night he would have blown a fuse.
"When I take office I intend to ask the city solicitor if I can tell councillors who I think are improperly dressed that they should retire from the meeting until they put on a jacket and tie."
Mr Gregson, the Hereford and Worcester branch secretary for the public sector union Unison, said he would continue to dress casually when it suited him. "It's not important what you wear or what you look like," he insisted.
"People judge you by what you say and do.
"There are a couple of us who have been known to turn up for meetings in the summer wearing shorts.
"I normally wear whatever I've got on during the day. If it's appropriate to wear a jacket and tie for work then I will but if it's not then I don't.
"I knew Les wouldn't appreciate the shirt but at least United play in red ...
"If we had not beaten Nottingham Forest I don't think I would have been seen wearing it in public but now United are quite definitely going to win the championship."