A foreign ministry panel confirmed Tuesday that secret pacts between Japan and the United States on nuclear arms existed in the Cold War era, DPA reported.
The first revelation by the Japanese government ended longtime official denial although declassified US documents had already confirmed such agreements.
Among the secret pacts acknowledged by the panel was a tacit agreement that allowed US vessels carrying nuclear arms into Japanese ports in violation of Japan's non-nuclear principles.
There has been a strong anti-nuclear sentiment among the Japanese public following the US atomic bombings of Hiroshima and Nagasaki in 1945.
Foreign Minister Katsuya Okada said he could not rule out the possibility that nuclear arms had entered Japan under the secret pacts.
"We cannot clearly state that there was no nuclear introduction to Japan. We cannot dispel doubts about it," Okada said.
The investigation started after the Democratic Party of Japan (DPJ) took power in September. Prime Minister Yukio Hatoyama promised to make government more open than under long-ruling Liberal Democrats, who repeated adamant denials of the existence of such pacts.
The DPJ won a landslide victory in the August election, ending more than a half-century of almost uninterrupted rule by the Liberal Democratic Party.
While Okada said he hoped the panel's report would help regain public trust in Japan's diplomacy, he still deplored the fact that the pacts had been hidden from the public for such a long time.
"Prime ministers and foreign ministers, as leaders, should be blamed" for the concealment, Okada said.