Last week, Prime Minister Shinzo Abe rejected a demand by the lawmakers' group for the government to conduct its own probe on the state's role in the military brothels, saying the government would cooperate as needed with a study by the party.

"We plan to start the new study by the end of the month," said former Education Minister Nariaki Nakayama, who heads the Liberal Democratic Party (LDP) group that denies victims' accounts of being forced by Japanese soldiers to work in the brothels.

"We need to speak out, otherwise we would be admitting to what others are saying," he told Reuters in an interview.

Abe sparked outrage abroad when he said earlier this month there was no evidence that Japan's government or army had forced women, mostly Asian and many of them Korean, to work in military brothels during World War Two.

But with furor from abroad threatening to cloud summits with Chinese and U.S. leaders, Abe has since repeated that a 1993 apology to the "comfort women", as they are euphemistically known in Japan, remained in effect and has expressed sympathy for them.

Abe's remarks precede a visit to Tokyo in mid-April by Chinese Premier Wen Jiabao and Abe's trip to Washington later that month.

LDP Secretary-General Hidenao Nakagawa also told Reuters this week that the party had no plan for a fresh probe.

Nakayama, however, said his group of about 130 lawmakers would press on, saying studies by academics and former soldiers showed that the government went too far in acknowledging coercion by the military and state in the 1993 apology.

"The government conducted a study before the apology was issued, but there were no documents then to show there was coercion by the government," he said, arguing that the phrase was used to appease South Korea ahead of a visit to the country by then-Prime Minister Kiichi Miyazawa.

The LDP group would ask the government for new documents, including surveys of former "comfort women" and would ask the government to review the 1993 apology depending on the findings, Nakayama said. Analysts say Abe's original remarks on the issue were intended to woo back his conservative base ahead of elections for parliament's upper house in July as opinion polls showed declines in support.