A privacy watchdog has ruled that intelligence unit National Security Agency’s (NSA) mass collection of citizens’ telephone data is illegal and alleged of violating civil liberties, with further demands to be shut down.

According to a latest report from the Privacy and Civil Liberties Oversight Board (PCLOB), the surveillance programme exposed by ex-NSA contractor Edward Snowden has "minimal" benefits in preventing terrorism

In addition, the Congress setup panel also claimed that there is no legal basis for the mass collection of telephone records, and concluded "we believe the programme must be ended."

"We have not identified a single instance involving a threat to the United States in which the program made a concrete difference in the outcome of a counterterrorism investigation," the panel said.

"The Section 215 bulk telephone records program lacks a viable legal foundation under Section 215, implicates constitutional concerns under the First and Fourth Amendments, raises serious threats to privacy and civil liberties as a policy matter, and has shown only limited value.

"As a result, the Board recommends that the government end the programme."

However, the Washington earlier argued it is legal to gather metadata on citizens’ phone calls under a section of the George W Bush-era Patriot Act that allows FBI to command from businesses information considered related to their enquiries.

"We have not identified a single instance involving a threat to the United States in which the program made a concrete difference in the outcome of a counterterrorism investigation,” the panel added.

"We are aware of no instance in which the program directly contributed to the discovery of a previously unknown terrorist plot or the disruption of a terrorist attack.”

The report also urges the government to erase data as soon as possible and limit access to citizens’ call records that are more than two degrees – and not three – from a suspect.

Recently, President Barack Obama ordered the spying agency to end collation of data on American citizens, following a series of disclosures regarding the extent of the agency’s spying powers, while the move has been criticised by Tech giants who slammed new restrictions as ‘insufficient’.