Fans of Manchester United are appealing for the return of a sentimental flag which they say was taken on the same day as their team played Sunderland.
A picture of two men has been posted on Twitter of two men holding a flag which reads "Champions of Europe since 1958" as part of an alleged prank.
The tweet is captioned: "Sunderland with stolen Manchester United flag yesterday #SAFC #MUFC."
One of the men is wearing a blue hat with the name of former Sunderland brewery company "Vaux" written on it.
Vaux sponsored SAFC in the 1980s and 90s.
The flag, said to have been taken from Sam Platts pub in Manchester, is believed to have been signed by a number of former MUFC players.
The flag is in tribute to the 23 people, including eight players, who died when a plane carrying the Manchester United squad on their way back from a European Cup tie crashed in February 1958.
United manager Matt Busby and England international Bobby Charlton were among those who survived the crash and managed to recover.
A number of fans writing on the Ready To Go forum have called for the return of the flag.
One user wrote: "Twitter reckons it was taken from outside a pub during the game.
"What's the point? Someone's stumped up cash to get it made, and what's one of our fans gonna do with it now, pathetic."
Another user posted: "Let's be polite and say it was a drunken prank, it can be resolved relatively easily, whoever knows them gets them told to get it sorted and returned damn quick."
Sunderland were beaten 3-1 at Old Trafford on Boxing Day, with the Black Cats' Fabio Borini bagging a spectacular late consolation goal.
A spokeswoman for Great Manchester Police said the force had not been received any reports of a stolen flag.