South Korea injected a record amount of 50,000 won banknotes into circulation last year, the central bank said Wednesday, despite a low return rate of the highest denomination bills that has continuously raised concerns over the possible growth of the underground economy.

In 2015, the Bank of Korea issued some 20.5 trillion won worth of 50,000-won notes, up 34.8 percent from the previous year and breaching the 20-trillion-won mark for the first time since it introduced the 50,000-won bill in 2009.

The increase was partly attributed to a rise in demand for the highest denomination bills due to the growth in the sheer size of Asia's fourth-largest economy, but also to an unusually low rate of return that possibly indicates a flow of large sums into the so-called underground economy.

The underground economy refers to undetected and unregulated business activities that also includes slush funds.

The return rate of 50,000-won bills came to 40.1 percent last year, compared with 25.8 percent from the previous year, according to the BOK.

However, the rate is still far lower than those of lower denomination bills at well over 80 percent.

Meanwhile, the central bank said it issued 14.38 trillion won worth of 10,000-won bills in 2015, down 12.3 percent from a year earlier while the amount of 5,000-won bills issued also dropped 5.9 percent on-year to 412 billion won over the cited period. (Yonhap)