TOKYO (Reuters) - Japanese Prime Minister Yoshihiko Noda on Wednesday clinched an 11th hour compromise with his opposition rivals to save his hard-fought deal on contentious sales tax increases, securing their backing in return for a pledge to call an election soon.

"We confirmed two things: first, to quickly pass the sales tax and social security reforms based on the three-party agreement, and second, once those reforms are passed to seek a mandate from the people in the near future," Noda told reporters.

Noda told reporters that he and leaders of two main opposition parties agreed to pass the tax bill in the final upper house vote and proceed to hold a general election.

Noda, who took over nearly a year ago as the Democratic Party's third leader since they surged to power in 2009, staked his political career on the plan to double the 5 percent sales tax by 2015 to curb Japan's ballooning debt.

He had appeared close to that goal after the Democrats forged a deal with the LDP and its former coalition partner in June.

But scenting possible victory given voter anger at the Democrats' failure to live up to their campaign pledges, the opposition has upped the ante, insisting on an early poll in return for backing the tax bill.

(Reporting by Sumio Ito and Shinji Kitamura; Writing by Tetsushi Kajimoto and Linda Sieg; Editing by Edmund Klamann)