Japan's parliament on Friday enacted a bill to bring about the country's largest ever change to the distribution and boundaries of lower house single-seat constituencies to correct a vote-value disparity that the Supreme Court has judged to be constitutionally problematic.

The bill, which was passed by the House of Councillors, aims at narrowing the vote disparity below the 2-fold level between densely and sparsely populated districts within the country.

The House of Councillors passes a bill to revise the distribution and boundaries of lower house single-seat consitutiencies at a plenary session in the parliament in Tokyo on Nov. 18, 2022. (Kyodo)

The revision to the Public Offices Election Act will add 10 single-seat electoral districts to five prefectures, while cutting one each from 10 prefectures, affecting 140 constituencies in 25 prefectures in total.

Under the amendment, 10 less-populated prefectures, including Wakayama and Yamaguchi, will lose one seat each, while five densely populated prefectures, such as Tokyo and Kanagawa, will obtain one to five extra seats.

Osaka and nine other prefectures, meanwhile, will retain their single-seat electoral districts, but their boundaries will be redrawn to reduce the disparities.

The amendment will also switch some seats allocated to the proportional representation system in the House of Representatives, in which seats are distributed in accordance with the total votes cast for each political party.

Among 11 proportional representation blocks nationwide, the revised law will cut one seat each from three blocks and give them to two other blocks.

Single-seat districts currently account for 289 out of the 465 lower house seats, while the remaining 176 are allocated through proportional representation.

The changes reflect recommendations presented by an expert panel to Prime Minister Fumio Kishida in June. Based on the national census conducted in 2020, they would bring down the maximum vote-value disparity from 2.096-fold to 1.999-fold.

The changes will take effect in voting in the next lower house election, which does need to be held until October 2025.

Vote-value disparities arise because of the different number of voters in constituencies.

Large disparities in the weight of votes cast have led to numerous lawsuits claiming they violate the Constitution. The supreme law stipulates that "all of the people are equal under the law and there shall be no discrimination in political, economic or social relations."

If a disparity is significant, the election is deemed to be in a "state of unconstitutionality" and will be ruled "unconstitutional" if the Diet does not address the issue within a reasonable period, according to legal experts.

The Supreme Court has ruled the lower house elections of 2009, 2012 and 2014 where the vote disparity was above 2.0-fold were constitutionally problematic, saying they were in a "state of unconstitutionality."