Wu Zhicheng, a Beijing resident who has no hukou, checks a paper that may help him reinstate his household registration in January. Wu said he lost his hukou 43 years ago because of a mistake that occurred when the local police relocated it to his employer. [Photo by Wu Jiang/for China Daily]

As highlighted in Wan's survey, cases with no hukou cover different situations. Apart from additional babies, children born out of wedlock and people who lost relevant certificates are also among the targeted group.

Earlier this year, two young people aroused people's attention by seeking donations of 40,000 yuan ($6,440) to cover the social maintenance fees of their baby at a crowdfunding website. The two were previously lovers but broke up when the woman was 17 weeks' pregnant.

Later, when the mother tried applying for a hukou for her newborn, she was required to pay a social maintenance fee and hand in the medical examination proving that the man was the biological father.

Social maintenance fees vary in different provinces, ranging from 30,000 yuan to 800,000 yuan.

Another type of case involves university graduates.

Students usually transfer their hukou from their hometown to university when starting campus life, and move it again after graduation. But sometimes graduates lose files needed for the transfer and then become one of those without a hukou.

The re-registration of a hukou took Wang Yuan (alias), a graduate from Guangzhou, South China's Guangdong province, more than six months.

A survey shows that except graduates, those without a hukou receive a lower level of education. About 44.2 percent are illiterate or have no formal education, and 30.7 percent only received elementary education.

According to Wan's survey, children born in violation of the family planning policy account for more than 50 percent of the total number with no hukou.

"The real number of household registrations is an important aspect of national conditions," said Lu Jiehua, a professor at Peking University, "For individuals, it's the pass for education, medical treatment and travel."