Outstanding balances on credit cards in the U.K. rose to £68.1 billion in April, an annual growth rate of 9.7%, the Bank of England said May 31.

The growth rate is the highest since February 2006, the Financial Times noted the same day, adding that the numbers are likely to add to BoE concerns that the increase is unsustainable and could be fueled because lenders are lowering their underwriting standards.

The growth rate stood at 6.8% in April 2016 and 5.3% in April 2015, the FT said.