Bradford Bulls have urged "true fans" to steer clear of rogue traders cashing in on the club's Challenge Cup success.
Street hawkers are dodging breach of copyright prosecutions by selling scarves, T-shirts and caps which did not feature the Bulls' logo.
Such sales starve the club of vital funds and can leave fans with shoddy goods, Bulls officials warned.
The warning comes following reports of traders operating in Bradford city centre - and even close to the Bulls own shops or Odsal Stadium.
Bulls Merchandising Manager John Hunt said: "If people have the club at heart they should buy from the Bulls' official outlets."
He added he had seen some goods sold by rogue traders which had been "falling apart at the seams". People who bought them had no way of getting a refund.
The salesmen have been cashing in on the latest wave of Bull-mania in the build up to Saturday's Murrayfield final against Leeds Rhinos.
"We had a fans' open day on Sunday and, as well as being able to get autographs from the players, visitors could pick up Bulls merchandise from marquees we had set up," said Mr Hunt.
He said one unofficial trader had set up shop outside Odsal Stadium, selling his wares to those going in and out.
"If they do not have a hawkers licence, the police can move them on, but that is all they can do," he added.
"They know that that they can use the words 'Bradford Bulls', the teams colours and everything else, as long as they do not include the club's logo."
If unofficial goods were found to boast the logo, the club were quick to take action, he added.
"In the last four years it has happened four or five times and we have told those involved we would sue them.
"We have not had to take further action because they have stopped," he said.
Mr Hunt warned the club kept a close watch to see that traders did not breach the club's copyright.
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