The revolt is a further blow for Prime Minister Naoto Kan, who has been in office less than a year and is battling approval ratings below 20 percent and the threat of legislative gridlock from the conservative opposition.

The rebels are supporters of Kan's nemesis, scandal-tainted faction boss Ichiro Ozawa, often dubbed the "Shadow Shogun", who failed in a bid last year to oust Kan, but who did vote with the government in the parliamentary vote.

The premier's top spokesman Yukio Edano warned that the 16 breakaway lawmakers "will be dealt with severely" by the Democratic Party of Japan (DPJ), which took power in a 2009 landslide but has since slipped badly in the polls.

"Those who missed the vote on purpose cannot possibly be accepted by the public as they can hardly be described as responsible members of the ruling party," Edano, the chief cabinet secretary, told reporters.

Kan earlier told reporters the boycott was "regrettable" and said "it is urgent to approve the budget and enact it for the sake of the people".

The premier faces tough hurdles in coming months as he seeks to ram economic stimulus measures through parliament and to find ways to whittle down a public debt mountain twice the size of Japan's five-trillion-dollar economy.

The 92.4 trillion yen budget for fiscal 2011 is almost certain to clear parliament before its April 1 deadline under Japanese constitutional rules, even if the opposition Liberal Democratic Party (LDP) blocks it.

However, bills required to enact and finance the budget are in doubt, because Kan's party lacks the two-thirds lower house majority to overcome opposition in the conservative-controlled upper chamber.

Crucial among those bills is one to issue 38.2 trillion yen worth of bonds to cover the deficit without which, observers warn, the government could be headed for a cash shortage within months.