Tickets for the Olympics have been found on eBay for at least three times their face value.
One seller wanted £3,783 for 26 tickets to events including archery, shooting, table tennis and weightlifting. Their original value was £1,140, the Metro online newspapers said.
The eBay sales website has pulled the listings.
Would-be spectators are urged to only buy tickets through official London sellers.
Touts could face a £20,000 fine if they resell London 2012 tickets.
A London 2012 spokesman said: "Customers are advised to purchase London 2012 Games tickets only through official sources.
"It is illegal to sell London 2012 Games tickets without the permission of Locog (the London 2012 organisers).
"Any website or individual claiming to have tickets for sale should be checked against the list of official outlets on the London 2012 ticketing website - http://www.london2012.com/about-this-website/stay-safe-online.php."