WASHINGTON - President Obama plans to deal with a Dec. 31 deadline that automatically would declassify secrets in more than 400 million pages of Cold War-era documents by ordering governmentwide changes that could sharply curb the number of government records hidden from the public.

In an executive order he is likely to sign before year's end, Obama will create a National Declassification Center to clear up the backlog of Cold War documents.

But the order also will give everyone more time to process the 400 million pages rather than flinging them open at year's end without a second glance.

The anticipated timing of Obama's order was disclosed by a government official familiar with the planning who requested anonymity in order to discuss the order before its release. A draft of the order leaked in the summer.

The order aimed at eliminating unnecessary secrecy also is expected to direct all agencies to revise their classification guides - the 2,000 manuals used by federal agencies to determine what information should be classified and what no longer needs that protection.

The manuals form the foundation of the government's classification system.

Two of every three such guides have not been updated in the last five years, according to the 2008 annual report of the Information Security Oversight Office, which oversees the government's security classification.

The still-classified Cold War records would provide a wealth of data on U.S.-Soviet relations, including the Soviet invasion of Afghanistan, the fall of the Berlin Wall, diplomacy and espionage. A Soviet spy ring in the Navy led by John Walker headlined 1985, which became known as "The Year of the Spy."

It took 19 years and a lawsuit for the National Security Archive, a private group that analyzes once-secret government records, to get documents on the 1959 crisis in which the United States and the Soviet Union faced off over control of West Berlin.

For nearly two decades, the contested documents were shuttled back and forth among various offices in the Defense Department, then on to the State Department and an unnamed intelligence agency. Each conducted a separate declassification review, before the government finally gave some of them up.

Obama's executive order will follow on the president's Inauguration Day initiatives on open government.

On his first day in office, Obama instructed federal agencies to be more responsive to requests for records under the Freedom of Information Act, and he overturned an order by President George W. Bush that would have enabled former presidents and vice presidents to block release of sensitive records of their time in the White House.

Obama's executive order "is an experiment, but it just might work," said Steven Aftergood, director of the Project on Government Secrecy at the Federation of American Scientists.

"By changing the rules about what gets classified, this could lead to a dramatic reduction in secrecy throughout the government." Aftergood obtained a leaked copy of an early draft of the executive order in the summer.