Britain's Sunday Times newspaper reported that several thousand tickets to the best events have been put up for sale by National Olympic Committee officials from their official ticket quotas.
Some events for the July 27-Aug. 12 games, such as the men's 100-meter track final, are listed at vastly inflated prices.
One of the most damaging allegations was against Spyros Capralos, the Greek Olympic Committee president and top organizer for the 2004 Athens Olympics.
Capralos was quoted as saying he had "pulled strings" with London organizing chairman Sebastian Coe to obtain an extra batch of premium tickets for official agents in Greece, on the pretext that demand in his country outstripped expectations.
The newspaper said Capralos told its undercover reporters that demand had actually been very low, and that many of the tickets were subsequently sold to people outside Greece for profit.
The London organizing committee said Capralos' alleged boasts of discussions with Coe were untrue.
But the Sunday Times said its undercover reporters, who posed as illegal ticket sellers acting for clients in the Middle East, caught officials red-handed. It said it has presented the IOC with a dossier of evidence on 27 officials controlling the tickets for 54 countries.
The Associated Press contributed to this report.