China's top population authority said Monday that over 30 percent of the country's migrant workers are employed without a proper contract to protect their rights.

Only 51.3 percent of migrants hold fixed-term contracts, and they work an average of 54.6 hours per week, far above the 40-hour legal limit, according to a report issued Monday by the National Population and Family Planning Commission.

China's floating population, those who have left their registered hometowns without obtaining a new household registration, or "hukou," reached 230 million in 2011, or 17 percent of the total population.

The group also experiences an inferior social security status than those with a hukou, as less than 30 percent of migrants workers have joined the social insurance systems at their work locations, which is a mandatory obligation, said the report.