BEIJING - Modern technology may be allowing someone who has not even followed you to know exactly where you have been.

When you use yikatong, the blue cards passengers often swipe when boarding a public bus or subway train, a person can learn exactly when you rode public transport and where you went simply by typing onto a website the 17 numbers appearing on the card.

Li Tiejun, an engineer at the Beijing office of Kingsoft, a Zhuhai-based software company, wrote about the card "secret" on his micro blog after discovering on Sunday that he could use the Beijing Municipal Administration and Communication Card Co's official website to obtain information about his previous whereabouts. He did not even have to bother with typing in a password to get it.

"I was shocked to see so many details about where I had been," he said. "I felt as if someone had followed me and knew a lot about me. It was annoying."

He compared the company's website to a big database without a lock.

Within 20 hours, Li's complaints had been forwarded more than 3,000 times, provoking a controversy online about whether putting such information out to the public violates cardholders' privacy.

"This information not only infringes upon their privacy, but also may undermine public safety," Li said.

"It could be used by someone who is ill disposed (toward others) in horrible ways. The database may also be an easy target for hackers."

Despite Li's contentions, China Daily reporters found they could not access the Beijing Municipal Administration and Communication Card Co's website. "It's because too many people are using it at the same time", said an employee of the company who did not want to be identified.

A 26-year-old student from the University of International Business and Economics, who would only state her surname, Ding, said one should not necessarily be concerned that such information is available to the public.

"The card is not issued under people's real names and I think few people will try to track others by using this yikatong information," she said, adding that those who have extramarital affairs should be careful.

Wang, manager of the Beijing Municipal Administration and Communication Card Co, who declined to state his full name, said cardholders whose privacy has been violated should blame themselves for that.

"The card is not used with real names and only provides transport information, so it has no relation to privacy rights," he said.

Some netizens said a code system should be used on the website, but the manager said the company will not consider heeding that suggestion.

Yi Shenghua, a Beijing-based lawyer at Yingke Law Firm, said the company may indeed not be violating residents' privacy by disclosing their transport information online but that the practice does present a temptation to prying eyes.

"Residents have the right to obtain their own transport information through the website, but that doesn't mean the company can release the information without any protective measures," he said, calling on the company to install a password system to prevent privacy infringement.

More than 45 million yikatong cards are now in use in Beijing. By swiping them in front of a sensor, passengers can enjoy a 60 percent discount on public bus fares. The cards can also be used at designated supermarkets, on long-distance bus lines, on expressway tollgates in Beijing and in public telephones on the capital's streets.

Chengdu residents, meanwhile, have found that the local public transport operator's website contains similar information about them. In Shanghai, though, the public can only learn the amount of credit left on a yikatong card.

Many mainland cities have issued the cards to make it easier for commuters to travel.