Parliament's Intelligence and Security Committee has made the most detailed public disclosure yet of Britain's electronic snooping abilities.
The agencies' surveillance powers have been under scrutiny since former NSA contractor Edward Snowden leaked details of spies' ability to monitor phone and online communications.
The legislators said the electronic spy agency GCHQ accesses "a very small percentage" of Internet traffic through the fiber-optic cables that carry communications.
The report said a small portion of that data is collected and even less is read though even that amounts to thousands of items a day.
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"It is clear to us that GCHQ do not conduct blanket surveillance," committee member Hazel Blears said. "It's not blanket and it's not indiscriminate."
The report's summary stressed that GCHQ "is not collecting or reading everyone's emails."
Blears said that bulk interception of Internet data "has exposed previously unknown threats or plots" but the report, portions of which are redacted, did not give details.
The report revealed that spies have "bulk datasets" containing "significant quantities of personal information about British citizens."
It said some staff have been disciplined or dismissed for "inappropriately accessing personal information in these datasets in recent years."
And they said there should be additional safeguards for sensitive professions including lawyers, doctors and journalists.