The data-collection program is set to expire by June 1, but the NSA will need to start the process to close it down sooner "to ensure that it does not engage in any unauthorized collection" of phone records, the Justice Department officials said in a memo to lawmakers.
The House approved a bipartisan measure last week to rein in the NSA program while still allowing some access to the telephone information law enforcement agencies say they need.
But the bill's fate in the Senate is uncertain. Republican leaders oppose the House measure and prefer to renew the NSA program as is.
The Justice Department warning noted that a court order stipulates the government must file a request for any renewal no later than Friday.
"For these reasons," the memo said, "after May 22, 2015, it will become increasingly difficult for the government to avoid a lapse in the current NSA program of at least some duration."
Separately, FBI Director James B. Comey has warned in recent days that congressional inaction would also put at risk other crime-fighting tools the bureau needs in the war on terrorism.
He said the NSA program allowed the FBI to get court orders for data on individual suspects, and to conduct surveillance of "lone wolf" suspects who are not linked to foreign terrorist groups, all of which would be barred if the program comes to an end. He said agents also would not be able to get roving wiretaps to follow suspects who use multiple phone devices.
Losing those tools, Comey said in a speech Wednesday at Georgetown University, would cause "a big problem."
Civil liberties groups counter that law enforcement agencies still have many tools at their disposal.
Sen. Rand Paul (R-Ky.) took to the Senate floor Wednesday in an hours-long address to voice opposition to renewing the NSA program.