The Supreme Court ruled Wednesday that the Upper House election in the summer of 2013 bordered on being unconstitutional due to an “outrageous” disparity in the weight of votes between districts, but it rejected the plaintiff’s demand to invalidate the result.

The ruling indicates again the judiciary’s growing impatience with lawmakers’ continued reluctance to rectify the inequality in vote values in national elections between heavily populated districts and their more sparsely populated counterparts.

The top court’s decision is likely to provide reform-minded forces with further momentum to press for a full-fledged revamp of the electoral system, even as the nation prepares for the snap Lower House election slated for Dec. 14.

Although the government took some steps to address the problem ahead of the 2013 campaign, the “disparity rate among constituencies that (dogged) the election was so egregiously high that it posed a threat to the election’s constitutionality,” Chief Justice Itsuro Terada said in Wednesday’s ruling.

In a stopgap bid to reduce such inequalities, the Diet in November 2012 — eight months before the July 2013 election — passed a bill to change the number of single-seat districts in four prefectures.

The Supreme Court decision on Wednesday apparently acknowledged this effort. Instead of just tweaking districts, the government “should pursue a legal revision to overhaul the current electoral system and eliminate the unconstitutionally high level of vote weight disparities,” the court said, in a further rebuke.

The ruling was delivered by the 15-member Grand Bench. Of the 15 justices, four voted to rule the Upper House election downright unconstitutional, including one who said its results should be nullified.

The ruling was handed down in a combined response to 16 lower-court lawsuits filed last year by lawyers who argued the 2013 Upper House election should be invalidated due to a vote value disparity that ran as high as 4.77. This means a vote in a sparsely populated district carried as much as 4.77 times more weight than a vote in a heavily populated constituency.

Following the election, the lead plaintiffs — attorneys Hidetoshi Matsunaga and Kuniaki Yamaguchi — spearheaded efforts to file the lawsuits nationwide over what they charge is the downright unconstitutionality of the 2013 election.

By the end of the year, all 16 courts had ruled more or less in their favor — none indicated the election was legitimate. Thirteen said the election was “in a state of unconstitutionality,” two said outright it was unconstitutional and one, the Okayama branch of the Hiroshima High Court, went so far as to declare it null and void.

Upper House elections by nature tend to result in higher vote value inequalities than those for the Lower House and frequently top the 5.00 threshold.

In scrutinizing Upper House elections, the Supreme Court has traditionally shown a lenient attitude, often condoning the disparity rate when it is less than 6.00.

But in one break with precedence, it ruled in 2012 that the 2010 Upper House election — in which the disparity rate was 5.00 — was “in a state of unconstitutionality,” apparently reflecting its simmering frustration with the government’s reluctance to rectify the inequality.