The National Security Agency has a secret backdoor into its vast databases under a legal authority enabling it to search for US citizens'email and phone calls without a warrant, according to a top-secret document passed to the Guardian by Edward Snowden.

The previously undisclosed rule change allows NSA operatives to hunt for individual Americans' communications using their name or other identifying information. Senator Ron Wyden told the Guardian the NSA's authorities provide loopholes that allow "warrantless searches for the phone calls or emails of law-abiding Americans".