Toxic loans in the Chinese financial system could be 10 times as high as official estimates suggest, Fitch Ratings has warned.

The international ratings agency said in a report on Thursday that, as a proportion of China's total loan pool, non-performing loans (NPLs) could be as high as 15-21 percent. By comparison, official data put the NPL ratio for commercial banks at 1.8 percent at the end of June 2016.

"There seems a high likelihood that banks' NPL ratios will continue rising over the medium term, in light of this discrepancy. There are already signs of stress, most obviously in the increased frequency with which banks are writing off or offloading loans, such as those to asset-management companies," the report said.

Solving China's bad loan problem would result in a capital shortfall of 7.4 trillion-13.6 trillion yuan ($1.1-2.1 trillion), equivalent to around 11-20 percent of China's economy, Fitch said.