Sheffield Wednesday's final game of the regular season against Wycombe at Hillsborough is expected to be a sell-out.
The club have confirmed 21,000 tickets have already been sold for a game that could see the Owls clinch automatic promotion.
Hillsborough ticket office manager Alastair Wilson said: "There was a high demand for this game prior to the weekend, but we've been inundated since the win over Carlisle on Saturday.
"The pattern so far is very similar to the Sheffield United game, where many supporters missed out on tickets, so I would advise our fans to ensure they purchase their tickets as soon as possible.
"We fully expect this game to completely sell out to Wednesdayites."
The Owls' 2-1 home win over Carlisle at the weekend, courtesy of Michail Antonio's winner in the fifth minute of stoppage time, coupled with Sheffield United's 1-0 defeat at MK Dons, leaves Wednesday a point behind their city rivals with two games to go.