The proportion of the middle class decreased sharply between 2005 and 2008 due to unemployment and an increase in household debt, a study shows.

In a report released Wednesday, the Hyundai Economic Research Institute said the ratio of the middle class to all households dwindled by 7.6 points from 57.5 percent in 2005 to 49.9 percent in 2008.

In accordance with OECD standards, the institute defined households with an average monthly income between 50 percent and 150 percent of the median value of income as the middle class.

During the same period, the proportion of the low-income class with income under 50 percent of the median value grew 4.9 points from 18.1 percent to 23 percent, but the wealthy class,  with income over 150 percent of the median value grew a mere 2.7 points from 24.4 percent to 27.1 percent.

The trend is attributed to a growth in unemployed people as a result of a drastic decrease in the manufacturing industry's ability to create jobs and a higher risk of household bankruptcy due to rising debt.

The National Statistical Office denied the institute's conclusions, saying a simple comparison between 2005 and 2008 is impossible because the institute's report on household income for 2005 did not take the income of one-person households into consideration. "The ratio of the middle class in 2008 stood at 65.5 percent, showing no conspicuous sign of dwindling," the NSO added.