The US government's authority to collect telephone records in bulk under legislation introduced after the September 11th, 2001 attacks lapsed at midnight (US time) after the Senate failed to agree on a new bill.
After an extraordinary Senate sitting on Sunday, the George W Bush-era spying measures exposed in 2013 by whistleblower Edward Snowden, a former National Security Agency contractor, will end temporarily.
Powers to continue collecting phone data belonging to millions of ordinary Americans with no connections to terrorism required fresh legislation as counterterrorism measures contained in the USA Patriot Act, which passed after the 9-11 attacks, were set to expire.
The government had used the Patriot Act to sweep up vast details on phone calls sparking a debate about privacy, infringement of civil liberties and government overreach in the name of national security.
On Sunday the Senate failed to agree on the USA Freedom Act, a new bill that would shift phone collection from the government to phone companies, or a short-term extension to the Patriot Act.
Last month the lower House of Representatives overwhelmingly passed the new legislation, which is supported by President Barack Obama, and the Senate was recalled early from a recess for a rare Sunday sitting to consider the new bill ahead of last night's expiration.
Senators voted late on Sunday to take up the House bill, 77 votes to 17, but Rand Paul, the Republican senator from Kentucky, blocked an extension of the NSA spying programme during a heated debate.
White House press secretary Josh Earnest said the Senate "took and important - if late - step forward" in advancing the Freedom Act but issued a warning about the powers lost with the lapsed authority.
“We call on the Senate to ensure this irresponsible lapse in authorities is as short-lived as possible,” he said.
“On a matter as critical as our national security, individual senators must put aside their partisan motivations and act swiftly.”
In another of his fiery speeches that have made him a national figure, Mr Paul, a 2016 presidential candidate and the most vocal critic of the domestic spying programme, conceded the new bill would pass in the coming days but threatened to delay it as long as possible.
“Little by little, we’ve allowed our freedom to slip away,” said the libertarian Republican on the Senate floor.
Mr Paul’s stance drew stinging criticism from fellow Republicans including the party’s leader in the Senate Mitch McConnell who accused Mr Paul of engaging in a “campaign of demagoguery and disinformation” prompted by the “illegal actions” of Mr Snowden.
“We shouldn’t be disarming unilaterally as our enemies grow more sophisticated and aggressive,” said Mr McConnell.
Republican Senator John McCain accused Mr Paul of endangering national security to raise his profile in the presidential campaign stakes.
Mr Paul remained defiant, posting a message on social media website Twitter after he singlehandedly ended the legal authority for the country’s intelligence agencies to track terrorists: “I came here to defend the Bill of Rights, not to be popular.”
The NSA began gathering phone records, including the details of numbers call and call durations, after the 9-11 attacks and was later granted the legislative power to continue collecting phone “metadata” under Section 215 of the Patriot Act which is now defunct following the midnight deadline.
The programme continued to operate in secret until it was revealed by Snowden, first in media leaks to the Washington Post and The Guardian. A news report was published about the vast dragnet in USA Today newspaper in 2006 but it went largely ignored.
