Half a million complimentary Expo tickets, which expire in three days, remain unused despite the public's relentless fervor toward the extravagant fair.

Statistics show that about 500,000 free tickets with the visiting period slated for July, or nearly one fifth of the free passes distributed to Shanghai households in the month under a package gift scheme, will become invalid on July 31.

A total of 2.3 million free tickets were distributed this month under the scheme, which aims to thank the residents of Shanghai who put up with years of inconvenience while Expo-related infrastructure was under construction.

The attendance at Expo 2010 Shanghai has continued to rise even as the sweltering heat baked the city.

As of Wednesday, or nearly half way through the six-month event, around 33.6 million visitors, most of them from the Chinese mainland, have visited the Expo since its opening on May 1.

In July alone, more than 12 million people passed through the Expo's turnstile, including 1.8 million of them holding the complimentary tickets.

Those with free tickets are advised to visit the Expo as soon as possible to avoid overcrowding in case too many people turn up to the Expo Garden at the end of each month.

In June, less than 100,000 complimentary tickets were unused, according to Expo organizers, while another 2 million new tickets will be issued under the scheme for August.

Hong Hao, bureau chief of Shanghai World Expo coordination, said that it's normal to have some unused tickets at such events.

At previous Expos, up to 10 percent of the total tickets that were distributed expired without being used, he said.

While some have kept the unused tickets as souvenirs, others are making quick and easy money by selling them at a rate lower than at authorized ticket booths.

But buying Expo tickets online may be risky, police warned, saying several fraud cases have been reported since early June.