Tickets for this weekend's Worthington Cup final at the Millennium Stadium are fetching up to £500 on the internet.
Fans are paying huge amounts for tickets
Sunday's clash between Liverpool and Manchester United in Cardiff has led to a scramble for tickets.
An investigation by BBC Radio Wales found three agencies charging hundreds of pounds for tickets which are being illegally sold on.
The Football League has said it is very concerned by the situation and fans who buy the tickets are being warned they will be refused entry to the match.
The match has long been sold out and fans anxious to get hold of tickets are prepared to pay over the odds to watch the match.
Internet ticket agencies are asking between £300-500 for seats with a maximum face value of £62.
Fans want to be at the Millennium Stadium
One agency told Radio Wales the tickets were bought from a network of worldwide brokers and were being offered at prices that reflected a premium which was passed on to fans.
Under the league's strict rules, clubs are told to keep records of who has bought tickets to avoid potential trouble.
The organisers and South Wales Police also want to be able to properly segregate different groups of fans.
Football League spokesman Ian Christen has advised fans not to buy the tickets, warning supporters risk not even seeing the match.
I have seen people being reduced to tears being turned away, £100 lighter."
Dave Holland, Cardiff trading standards officer
Mr Christen said fans who bought tickets for the wrong side of the stadium would be refused entry to the stadium.
He added some agencies were offering tickets they did not possess simply to push up prices.
Under current rules, buying a black market ticket is not a criminal offence, said Mr Christen.
Dave Holland, Cardiff Trading Standards officer, said officials would also be active to deter street ticket touts.
"Ticket touts cause a lot of problems. People get up to the gates and find they are fakes. We will be looking to take these touts off the streets.
"I have seen people being reduced to tears being turned away, £100 lighter."