Led Zeppelin fans have been told they may be refused entry to the band's reunion gig if they paid for tickets using someone else's credit card.
Robert Plant will perform with Jimmy Page and John Paul Jones
Several thousand people won a password allowing them to purchase tickets in a lottery draw earlier this week.
But promoters now say that tickets will be cancelled if the password holder's name is not the same as the one on the card used to pay for the tickets.
Fans have reacted angrily, claiming the terms changed after they made payments.
They say it was not made clear that the passwords were non-transferable in the initial email sent to ballot winners.
"There were no stipulations in respect of who could pay for, and thus claim, the tickets," wrote Dandu, from Canada, in an email to the BBC.
A second email, clarifying the terms and conditions, was apparently sent out several hours later - by which time many fans had already bought their tickets.
Promoter Harvey Goldsmith has defended the move on his website.
"It is painfully obvious that if the ticket is not transferable then the method of obtaining the ticket is not transferable either," he wrote.
Tickets for the show, on 26 November at London's O2 arena, cost £125 each, plus booking fee.
It will see the three remaining members of Led Zeppelin play together for the first time since 1988.
Pete Townshend, Bill Wyman and Paolo Nutini will also perform at the show, which is a tribute to Ahmet Ertegun, the late founder of Atlantic Records.
Goldsmith said the strict rules surrounding the ticket sales were intended to deter touts charging inflated prices for the eagerly-anticipated gig.
"If you think that you can beat the system by buying from eBay or any other website you are wrong, you will not be allowed in to the concert and your application for tickets will be cancelled within the next 14 days," he said.
"I have no interest in supporting parasite businesses who prevent fans from supporting their artists by the increased price of the tickets and who ultimately put nothing whatsoever back into our business to support it."
But fans have challenged Goldsmith's comments, saying the rules target "genuine fans" who do not own a credit card.
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"I used my mother's credit card," wrote Jimmy on the promoter's website. "It has the same billing address and surname as myself.
"I will be able to produce this credit card when picking up the tickets, which was the only stipulation spelled out in your original email.
"I contacted Ticketmaster and they have told me that my tickets will likely be cancelled."
Another fan, using the name meerlorre, added that he could not legally own a credit card because he was under 18 - but had not been prevented for registering for the ticket ballot.
However, Goldsmith advised fans facing such dilemmas to "contact us and give us a justified explanation" in order to retain their tickets.