Hundreds of tickets for the FA Cup semi-final tie between Manchester City and Manchester United have been stolen from a Royal Mail van.
Two special delivery bags, containing about 900 tickets, were taken from the van near the City of Manchester stadium on Friday evening, police said.
They had been collected from Eastlands for delivery to City fans before the game at Wembley on 16 April.
A Royal Mail worker has been arrested on suspicion of theft and bailed.
It is believed Greater Manchester Police raided two properties on Tuesday morning but no arrests were made.
The 39-year-old man was arrested soon after.
In a statement on its website, Manchester City said: "All supporters impacted by the theft will be contacted by the club in the coming days.
"Supporters are strongly advised not to purchase tickets from unofficial sources.
"Only tickets purchased direct from the club will be valid for the game."
A spokesman for Greater Manchester Police said: "Shortly before 7.50pm on Friday 1 April 2011, police were called to Ashton New Road, Manchester, following reports two bags of special delivery mail had been stolen from a Royal Mail van.
"A number of tickets for the FA Cup semi-final were stolen and GMP is working with Royal Mail and MCFC.
"Inquiries are continuing."
A spokesman for Wembley Stadium said the stolen tickets could not be used for the game.
He said: "The original tickets that were stolen have been cancelled on the stadium's access control system.
"Only the tickets for these specific seats that are now being re-issued to Manchester City will be valid on the day."
"The stadium will continue to work very closely with the Metropolitan Police Service in planning for this event."