The surveillance powers of US security services have been restricted after key parts of the Patriot Act expired at midnight.
The Senate's failure to reach a deal before the deadline means that security services have temporarily lost the legal provision to collect phone records from millions of Americans, to carry out "roving wiretaps" of terror suspects and to monitor "lone wolf" suspects, reports CNN.
However, these expired provisions are likely to be replaced with new legislation known as the Freedom Act, which would allow for new forms of data collection, while limiting the National Security Agency's sweeping surveillance programme revealed by Edward Snowden in 2013.
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The White House has described the Senate's failure to reach a deal in time as an "irresponsible" lapse. "On a matter as critical as national security, individual senators must put aside their partisan motivations and act swiftly," it said in a statement. "The American people deserve nothing less."
Its views were echoed by Director of National Intelligence James Clapper, who warned that the US "would lose entirely an important capability that helps us identify potential US based associates of foreign terrorists."
But critics remain unconvinced, with privacy campaigners arguing that a lapse in the controversial provisions would not affect the government's ability to conduct targeted investigations or combat terrorism.
"The government has numerous other tools, including administrative and grand jury subpoenas, which would enable it to gather necessary information," says the American Civil Liberties Union.
The expiration of surveillance authority represents a "profound shift" in American attitudes since 9/11, when counter terrorism and national security topped the agenda for both parties, says The New York Times.
"[Now], a swell of privacy concerns stemming from both the vast expansion of communication systems and an increasing distrust of government’s use of data has turned those concerns on their head," says the newspaper.
Meanwhile, the British government is coming under pressure to limit police access to private phone and email records after an increase in requests was revealed, The Guardian reports.
A report by privacy campaigners at Big Brother Watch revealed that police officers were making requests to access private communication records every two minutes and applications were being approved in 93 per cent of cases.
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