The US National Security Agency is collecting telephone records of millions of Verizon Communications customers, according to a secret court order obtained and published by the Guardian newspaper's website.
The order, marked "Top Secret", was issued by the US Foreign Intelligence Surveillance Court.
It directs Verizon's Business Network Services Inc and Verizon Business Services units to hand over electronic data including all calling records on an "ongoing, daily basis" until the order expires on 19 July.
Signed by Judge Roger Vinson at the request of the FBI, the order covers each phone number dialled by all customers and location and routing data.
It also covers the duration and frequency of the calls, but not the contents of the communications.
The disclosure comes with the Obama administration already under fire on other privacy and First Amendment issues.
In particular, it is being criticised for a search of Associated Press journalists' calling records and the emails of a Fox television reporter in leak inquiries.
Officials at the White House and the NSA declined to comment, as did Verizon spokesman Ed McFadden.
Verizon's biggest rival, AT&T Inc, did not provide any immediate comment when asked if the government had made a similar request for its data.
"That's not the society we've built in the United States," said Kurt Opsahl, an attorney at the Electronic Frontier Foundation, which is suing the NSA over surveillance inside the country.
"It's not the society we set forth in the Constitution, and it's not the society we should have."
The order expressly compels Verizon to turn over both international calling records and strictly domestic records, and it forbids disclosure of the order's existence.
It refers to mobile and landline numbers, though not explicitly to Verizon's consumer business.
The order is the first concrete evidence that US intelligence officials are continuing a broad campaign of domestic surveillance that began under former president George W Bush and caused great controversy when it was first exposed.
In 2005, the New York Times reported that the NSA was wiretapping Americans without warrants on international calls.
Los Angeles Times and USA Today later reported that the agency also had unchecked access to records on domestic calls.
In addition, a former AT&T technician, Mark Klein, said that a room accessible only with NSA clearance in the carrier's main San Francisco hub received perfect copies of all transmissions.
Privacy lawsuits against the government are continuing, though cases filed against the phone carriers were dismissed after Congress passed a 2008 law immunising the companies that complied with government requests.
That law also allowed for broader information-seeking, though methods must be approved by the special court handling foreign intelligence matters.
The new order cites legal language from the 2001 US Patriot Act, passed soon after the 11 September attacks, that allows the FBI to seek an order to obtain "any tangible thing" including business records, in pursuit of "foreign intelligence information".
Verizon is the second biggest US telephone company behind AT&T in terms of revenue.
The vast majority of Verizon's overseas operations come from its acquisition of MCI Communications, which is also covered by the order although foreign-to-foreign calls are exempted from it.
Mr Opsahl said it was unlikely that Verizon would be the only subject of such an order and that the other major carriers probably had similar orders against them.
It is unclear what the NSA and FBI do with the phone records they collect.
If past practices have continued, though, Mr Opsahl said, they are probably mined with sophisticated software in an attempt to figure out close connections between people the agencies consider to be terrorism suspects and their associates.