China’s biggest banks more than doubled the level of bad loans they wrote off last year, in a sign that financial strains are mounting as growth in the world’s second-largest economy slows.

The five biggest Chinese banks, which account for more than half of all loans, wrote off Rmb59bn ($9.5bn) in debts that could not be collected, according to their 2013 results. That was up 127 per cent from 2012, and the highest since the banks were rescued from insolvency, recapitalised and publicly listed during the past decade.

The sharp acceleration in write-offs is the latest indication of the turbulence now buffeting China’s financial system. The bond market suffered its first true default in March, two high-profile shadow bank investment products were spared from collapse by last-minute bailouts this year, and a small rural lender suffered a brief bank run last week.