The hidden debt of China's pension fund is estimated to be more than 20 trillion yuan ($3.3 trillion), China Securities Journal newspaper reported Thursday, citing Zheng Bingwen, director of the Centre for International Social Security Studies at the Chinese Academy of Social Sciences.

There has been no official data measuring the debt so far.

The National Audit Office has been advised by Zheng to conduct an internal investigation into the size of the hidden debt and future gap of the country's pension fund, which were not included in the figures for China's total government debt released by the State auditor on Monday.

The country's total government debt reached 30.27 trillion yuan by the end of June.