Tickets to watch Premier League leaders Leicester’s final home game are being offered for £15,000 a pair through an online re-sale website.
The 7 May match against Everton, which could see the Foxes become champions, sold out in 90 minutes on Monday.
But within hours, individual tickets were being advertised for more than £3,000 each and a pair for much more.
The club said it would “take action against any ticket touting” or attempts to “resell tickets above face value”.
Leicester are seven points clear of nearest rivals Tottenham with five games left. They have already secured Champions League football and need three more wins to guarantee the Premier League title.
They face West Ham and Swansea at their King Power Stadium then Manchester United at Old Trafford, before the game against Everton next month, followed by the final match of the season away to last year’s champions Chelsea on 15 May.
Leicester posted on their own website that the Everton game had seen “unprecedented demand” and sold out in “record time”, with the “vast majority” of higher-tier members able to get a ticket.
Membership does not guarantee a ticket, the club told the Leicester Mercury.
“The unauthorised sale of football tickets is a criminal offence under UK law,” added a club spokesman.