WASHINGTON - President Bush has quietly claimed sweeping new powers to open Americans" mail without a judge"s warrant, the Daily News has learned.

The President asserted his new authority when he signed a postal reform bill into law on Dec. 20. Bush then issued a "signing statement" that declared his right to open people"s mail under emergency conditions.

That claim is contrary to existing law and contradicted the bill he had just signed, say experts who have reviewed it.

Bush"s move came during the winter congressional recess and a year after his secret domestic electronic eavesdropping program was first revealed. It caught Capitol Hill by surprise.

"Despite the President"s statement that he may be able to circumvent a basic privacy protection, the new postal law continues to prohibit the government from snooping into people"s mail without a warrant," said Rep. Henry Waxman (D-Calif.), the incoming House Government Reform Committee chairman, who co-sponsored the bill.

Experts said the new powers could be easily abused and used to vacuum up large amounts of mail.

"The [Bush] signing statement claims authority to open domestic mail without a warrant, and that would be new and quite alarming," said Kate Martin, director of the Center for National Security Studies in Washington.

"The danger is they"re reading Americans" mail," she said.

"You have to be concerned," agreed a career senior U.S. official who reviewed the legal underpinnings of Bush"s claim. "It takes Executive Branch authority beyond anything we"ve ever known."

A top Senate Intelligence Committee aide promised, "It"s something we"re going to look into."

Most of the Postal Accountability and Enhancement Act deals with mundane reform measures. But it also explicitly reinforced protections of first-class mail from searches without a court"s approval.

Yet in his statement Bush said he will "construe" an exception, "which provides for opening of an item of a class of mail otherwise sealed against inspection in a manner consistent ... with the need to conduct searches in exigent circumstances."

Bush cited as examples the need to "protect human life and safety against hazardous materials and the need for physical searches specifically authorized by law for foreign intelligence collection."

White House spokeswoman Emily Lawrimore denied Bush was claiming any new authority.

"In certain circumstances - such as with the proverbial "ticking bomb" - the Constitution does not require warrants for reasonable searches," she said.

Bush, however, cited "exigent circumstances" which could refer to an imminent danger or a longstanding state of emergency.

Critics point out the administration could quickly get a warrant from a criminal court or a Foreign Intelligence Surveillance Court judge to search targeted mail, and the Postal Service could block delivery in the meantime.

But the Bush White House appears to be taking no chances on a judge saying no while a terror attack is looming, national security experts agreed.

Martin said that Bush is "using the same legal reasoning to justify warrantless opening of domestic mail" as he did with warrantless eavesdropping.