TOKYO (Reuters) - Japan's top court on Wednesday held unconstitutional some district polls in the 2012 election that brought Prime Minister Shinzo Abe to power because of wide gaps in the weight of rural and urban votes, but stopped short of invalidating the result, the country's media reported.

Holding the elections invalid could have sparked political chaos since no precedent exists, but few had expected the Supreme Court to take that stance since the top judiciary is not known for rocking the establishment boat. Wednesday's court decision leaves the issue of reforms in lawmakers' hands.

Noting that changes made since last year's election had not addressed fundamental problems with distribution, the court urged parliament to tackle further reform.

"It is necessary to continue to steadily deal with the issue of fixing the electoral system," Kyodo news agency quoted the court as saying.

Critics say Japan's electoral system gives more influence to rural voters, many of whom are elderly, than to younger city dwellers, so driving politicians to push for policies favoring welfare and protectionism over economic growth.

The system has also been credited with keeping Abe's Liberal Democratic Party (LDP) in power for most of the past 60 years.

"Election reform is a bedrock issue for Abenomics," wrote Robert Feldman, chief economist at Morgan Stanley MUFG in Tokyo, ahead of the ruling.

"Without electoral reform that ends the major disparities of voter weights, the election incentives that have created and preserved vested interests would not change. Hence, both economic and fiscal reform would remain extremely difficult."

Abe told reporters he took the ruling seriously and would now examine it more closely.

DISPARITIES PERSIST

High courts ruling on 16 lawsuits in March said elections in 31 of the 300 single-seat districts for parliament's 480-member lower house last December were unconstitutional, or held in a "state of unconstitutionality", because of wide disparities in vote weights.

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A single vote in the least populous district carried 2.43 times the weight of one in the most heavily populated district.

Two courts also took the unprecedented step of declaring elections invalid in three constituencies.

Wednesday's Supreme Court ruling covered all those 16 suits.

The Supreme Court had already ruled in 2011 that because one vote in the least populous district effectively carried the weight of more than two votes in the most populous constituency, the 2009 election was held in a "state of unconstitutionality", but similarly declined to rule the poll invalid.

After that ruling, lawmakers passed a bill to redress the imbalance by cutting one seat from each of five sparsely populated districts.

This reduced the maximum disparity to just under two, but the measures had yet to be finalized when a snap election was called. Redistricting has since been completed but disparities remain and have widened in some cases.

More drastic reforms are needed to reduce the excess clout of vested interests such as farmers and the elderly and ease the way to reforms to generate economic growth, experts said.

But some analysts said the outlook for meaningful change remained dim. "They continued the previous warning and didn't push it up a notch. They are still leaving it to the legislature," said Sophia University professor Koichi Nakano.

Abe won high marks for the first two steps of his "Abenomics" strategy, hyper-easy monetary policy and fiscal spending.

But investors have been disappointed with the failure of his so-called "Third Arrow" to target structural reforms and deregulation of sectors such as labor and farming.

Lawyers' groups have filed separate suits to invalidate the results of last July's election for parliament's upper house, in which the maximum vote value disparity was 4.77.

Abe's ruling bloc won that election, resolving a parliamentary deadlock that had hobbled policy steps.

(Reporting by Linda Sieg; Editing by Clarence Fernandez)