According to the security oversight office, federal departments classified 15.6m documents last year, twice the number in 2001, with the help of new categories with unclear functions such as "sensitive security information".

Meanwhile declassification has slowed from 204m pages of documents released to the public in 1997 to just 28m pages last year.

"You'd just be amazed at the kind of information that's classified - everyday information, things we all know from the newspaper," the former Republican governor of New Jersey and head of the 9/11 commission, Thomas Kean, told the New York Times. "We're better off with openness. The best ally we have in protecting ourselves against terrorism is an informed public."

Groups seeking greater freedom of information cited as examples of excessive secrecy: the Defence Intelligence Agency's removal of the fact that the former Chilean dictator Augusto Pinochet was interested in "fencing, boxing and horseback riding"; the justice department's determination to black out a four-line quotation from a published supreme court decision; and the CIA's legal battle to preserve the secrecy of its budgets from the 1950s and 60s.

In the past many of these documents would have been freely available. But since September 11 President George Bush has given new departments the power to classify documents, while creating vague new classes of classified information such as "law enforcement sensitive" and "homeland security sensitive".

"We find there's such a proliferation of these bogus categories, which lack clear rules or definitions," said Lawrence Halloran, aide to a Connecticut Republican who held a hearing on excessive secrecy in March.

Last week a Texas Republican and a Vermont Democrat co-sponsored a bill to ensure that any new exemptions under the freedom of information act would have to be explicitly disclosed.

A rare insight into some of the information the state has refused to release came in 2002 when the DIA released a biography of Gen Pinochet with half the text deleted, three years after releasing the whole thing. Information cut included the remark that "General Pinochet is conservative in his political thinking".

A spokesman for the National Security Council said the huge increase in classified material was primarily a result of the huge increase in documents spawned by email. "The administration is proud of its record of openness," he said.