Japan's ruling party has extended term limits for its leaders, a change that gives Prime Minister Shinzo Abe a shot at becoming the country's longest-serving post-war leader.

On Sunday, the ruling Liberal Democratic Party (LDP) congress approved a proposal that extends the limit to three consecutive three-year terms, up from the previous two consecutive three-year term limit.

This means Abe can stand for re-election in the next party leadership vote in 2018.

If he wins the party election and the national polls, he could remain in power until September 2021.

Under the previous limit, Abe would have had to step down as party leader and prime minister in September 2018, even if the LDP was still in power.

The current longest-serving prime minister in the post-war era is Eisaku Sato who served from November 1964 to July 1972.

Abe, 62, served as prime minister for about a year before stepping down following a historic defeat in upper house elections in 2007.

He became prime minister again when the LDP returned to power in December 2012 after a three-year period in opposition.

He then launched his "Abenomics" growth plan - a mix of massive monetary easing, government spending and red-tape-slashing, but five years on, growth remains fragile and inflation well below the target of two percent.

The LDP also adopted a policy principle of "taking a practical step towards proposing amendments to the constitution".

The party has long sought to revise the post-war constitution that bans Japan from the use of force except in the strictest sense of self-defence.

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The constitution was imposed by occupying US forces after World War II and took effect in 1947.

Abe, however, faces some controversy over an alleged land deal involving a primary run school linked to his wife, Akie.

The deal involves the bargain-price sale of public land to the controversial new school, where the Japan's first lady briefly held a role as honorary principal.