By Rie Ishiguro and Stanley White
TOKYO (Reuters) - Japan's government on Friday will submit laws to double its sales tax by 2015 to fund swelling social security costs in the world's fastest-ageing nation, setting up a showdown that could split the ruling party, force early elections and deepen policy paralysis.
Prime Minister Yoshihiko Noda has staked his political career on the tax hike plan, but the chance of success looks slim as opposition parties in a divided parliament baulk at cooperating with the government.
Opposition to the tax hike also remains strong among the ruling Democrats, who this week spent long hours thrashing out minor changes to the legislation to win over dissenters.
The government will submit the bills to parliament on Friday, although Noda has yet to find a way to win backing from the opposition, which controls the upper house and has threatened to block the bill to force an early election, which does not need to take place until late 2013.
Members of the ruling party could also revolt and vote against the bill, which calls for the 5 percent sales tax rate to be raised to 8 percent in April 2014 and then to 10 percent in October 2015.
The International Monetary Fund and other organizations, rating agencies and many economists have repeatedly warned the clock was ticking for Japan, saddled with public debt twice the size of its $5 trillion economy and borrowing more than it raises in taxes.
The risk is that while Japan today can still satisfy all of its borrowing needs at home and at very low rates, delays caused by the political gridlock could push it to a point of no return.
Japan spends half of its tax income just to service its debt. Each year it adds more than the combined gross domestic product of Greece and Portugal to its debt pile.
In a sign of the precarious state of Japan's public finances, new borrowing in the fiscal year starting in April is expected to exceed tax revenue for the fourth year in a row.
"The road ahead will be quite rocky... but we'll step up efforts to enact the legislation quickly in order to ensure social security and restore public finances," Finance Minister Jun Azumi said after cabinet meeting.
Noda, who wants the bills passed into law during the current parliament session that ends in June, is due to meet the press at 6 p.m. on Friday. (0900 GMT)
The People's New Party, a coalition partner of the Democrats, has threatened to leave, arguing it was part of their agreement not to raise the tax when the parties formed the coalition in 2009.
That threat is not too significant given that the People' New Party has just eight seats in both chambers of a 722-strong parliament and its departure would not topple the government.
Noda's greater concern is keeping his party together given that many Democrats fear voter backlash given the drubbings ruling parties suffered in past elections when even just the possibility of raising taxes was mentioned.
Japan, the world's oldest major society, expects two out of every five of its citizens to be 65 or older by 2060, reflecting low birthrates and long life expectancy, driving up welfare costs and shrinking the tax base.
(The story has been corrected in headline and text to make clear that laws have not yet been submitted, but will be submitted later on Friday)
(Additional reporting by Tetsushi Kajimoto; Editing by Tomasz Janowski and John Mair)