TOKYO — Japan confirmed for the first time today the existence of once-secret Cold War-era pacts with the U.S. that tacitly allowed nuclear-armed warships to enter Japanese ports in violation of Tokyo's postwar principles.

While declassified U.S. documents have already confirmed such 1960s agreements, Tuesday's revelation broke with decades of official denials.

The investigation by a government-mandated panel is part of Prime Minister Yukio Hatoyama's campaign to rein in the power of bureaucrats and make his government, which was elected to power last year, more open than that of the long-ruling conservatives, who repeatedly denied the existence of such pacts.

The panel examined documents surrounding four pacts, including Tokyo's tacit permission that U.S. nuclear-armed warships could make calls at Japanese ports — a violation of Japan's so-called three non-nuclear principles not to make, own or allow the entry of atomic weapons.

There is strong aversion to nuclear weapons in Japan, the only country to suffer atomic bombings — in Hiroshima and Nagasaki at the end of World War II.

Speculation about such secret agreements have been swirling in Japan for years so the panel's findings most likely will simply confirm public suspicions rather than shock or anger people. Some are also aware of U.S. documents about these matters.