Japan ranked second-worst among advanced economies in 2000 in terms of the relative poverty rate because nonregular workers with low wages rose amid the prolonged economic slump, the Organization for Economic Cooperation and Development said Thursday.

The portion of the population living in relative poverty, defined as having less than half of the median household disposable income, was 13.5 percent in 2000, the second-highest among OECD members, following 13.7 percent in the U.S., the OECD said in a survey on the Japanese economy.

"Rising income inequality and the increasing proportion of the population in relative poverty threaten to weaken the consensus for further economic reforms," the OECD warned, dedicating an entire chapter to Japan's disparity issue in the report for the first time.