The political battle over the government-backed postal reform plan took a new twist Monday when Prime Minister Junichiro Koizumi rebutted an accusation by the main opposition party that he had consumed alcohol before a key House of Representatives plenary session last week.

The accusation came after the Liberal Democratic Party fired a similar volley at the Democratic Party of Japan late Friday.

On Friday, DPJ President Katsuya Okada claimed that some Liberal Democratic Party members, including Koizumi, attended the day's Lower House plenary session with red faces, an indication they had been imbibing.

The allegations were no laughing matter for Koizumi.

On Monday, Koizumi submitted a formal protest to the DPJ, saying he "resolutely protests" Okada's "groundless remarks (that) tarnished my honor."

"I did not drink even a drop of sake or any other alcoholic beverage that day. (The DPJ) has become an irresponsible party," Koizumi said Monday morning at the Prime Minister's Official Residence.

Friday's session, which convened at around 9 p.m., was held to vote to extend the ordinary Diet session by 55 days to secure time to deliberate the contentious postal privatization bills. The DPJ opposes both the bills and the extension.

On Sunday evening, the DPJ submitted a motion to Lower House speaker Yohei Kono demanding that the prime minister and three other LDP members be punished. The move came on the heels of a similar motion submitted late Friday night against two other LDP members.

LDP members on the Lower House Steering Committee reportedly admitted Friday that "some LDP members" had had drinks shortly before the session, but claimed "some DPJ members" had also shown indications of having consumed liquor. Later Friday, the LDP submitted a punishment motion to the chamber against three DPJ members.