In what is being seen as a blow to U.S. President Barack Obama’s defence of the National Security Agency’s broad surveillance programmes, an independent executive branch board of experts has said that the government’s collection of telephone ‘metadata’ is illegal and needs to be stopped.

Based on leaked copies of a 238-page report to be issued this week by the Privacy and Civil Liberties Oversight Board (PCLOB), media outlets here quoted Board members saying that the statute upon which the programme was based, Section 215 of the U.S. Patriot Act, “does not provide an adequate basis to support this programme.”

In particular the Washington Post reported that the PCLOB, which was established by the U.S. Congress in 2004 to advise the executive branch on privacy and civil liberties concerns in the context of counterterrorism policies, said, “We have not identified a single instance involving a threat to the U.S. in which the telephone records programme made a concrete difference in the outcome of a counterterrorism investigation.”

Among the numerous criticisms of the global mass surveillance policies of the NSA is the argument that it failed to discover the relatively unsophisticated plot behind the April 2013 Boston Marathon bombing, allegedly undertaken by two brothers with links to Chechnya.

This week the PCLOB argued, “We are aware of no instance in which the programme directly contributed to the discovery of a previously unknown terrorist plot or the disruption of a terrorist attack.”

The report comes in the wake of a speech by Mr. Obama last week, announcing limited reform of the NSA’s surveillance programmes, including introducing a measure of transparency and committing to restrict snooping on leaders of friendly nations such as German Chancellor Angela Merkel.

The latest discussions on NSA reform follows a string of exposés on the extent of spying that the Agency carries out, revealed by whistleblower and former NSA contractor Edward Snowden, currently under temporary asylum in Russia.

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