TOKYO — The unpopular government of Prime Minister Yoshihiko Noda suffered another setback Monday when the largest faction of his Democratic Party quit over a proposed tax increase, leaving the party barely in control of Parliament’s lower house.

Ichiro Ozawa, a former party head and its onetime power broker, led 49 other lawmakers in resigning from the party to oppose a bill backed by Noda to double the national sales tax to 10 percent by 2015.

While the prime minister said the increase was needed to defray the costs of Japan’s rapidly aging population, Ozawa called it a betrayal of the party’s pledge not to raise taxes, made before a historic election swept it to power three years ago.

The defection left the Democrats with only 251 of the 480 seats in the lower house, the more powerful of Parliament’s two chambers because it selects the prime minister.

This could worsen Japan’s chronic political paralysis by weakening the Democrats’ grip on Parliament, when the opposition already controls the upper house.

It also appears likely to further erode the already lagging popularity of Noda’s Democrats.

Noda joined forces with the Liberal Democrats, Japan’s largest opposition party, to win passage of the tax increase in the lower house last week.

Approval in the upper house is expected soon.