'Leaked report' reveals mass data fears
Spy officials allegedly voiced concerns back in 2010 that so much data was being collected by the UK security services, they risked overlooking useful intelligence.
The draft report, apparently written for the UK government, was obtained by the Intercept website from US whistle-blower Edward Snowden.
It suggests that "life-saving intelligence data" could be missed.
Its release comes as the Investigatory Powers Bill goes through Parliament.
Extracts from the the document read: "The security service... can currently collect significantly more than it is able to exploit fully.
"This creates a real risk of intelligence failure from the service being unable to access potentially life-saving intelligence from data that it has already collected."
The report is marked classified and dated 12 February 2010.
It was allegedly prepared by British spy agency officials to brief the government's Cabinet Office and Treasury Department about the UK's surveillance capabilities.
Those capabilities are currently due to be updated via the controversial Investigatory Powers Bill, which is now at the report stage in Parliament.
It aims to give legal backing to the bulk collection of internet traffic, as well as requiring service providers to store browsing records for 12 months.
The government said these added powers were necessary in the fight against terrorism
A revised version was drawn up earlier this year after a raft of concerns about whether it had got the balance between privacy and security right.
The government needs the bill to go through before the end of year, when the current laws regulating surveillance expire.
Critics of the bill said the alleged leaked document showed mass surveillance was not the answer.
Open Rights Group communications director Pam Cowburn told the BBC: "We have been calling on the government to make the operational case for costly bulk surveillance programmes that allow the collection of vast amounts of data.
"As today's leaks show, there are genuine concerns that mass surveillance is making us less, not more, safe.
"If the Investigatory Powers Bill is passed, even more of our data will be collected, with internet service providers being forced to record our web browsing history and app use.
"Surveillance should be targeted towards people who are suspected of criminal activity, not the entire UK population."
GCHQ told the BBC: "It is long standing policy that we do not comment on intelligence matters.
"Furthermore, all of GCHQ's work is carried out in accordance with a strict legal and policy framework, which ensures that our activities are authorised, necessary and proportionate, and that there is rigorous oversight, including from the secretary of state, the interception and intelligence services commissioners and the Parliamentary Intelligence and Security Committee.
"All our operational processes rigorously support this position. "
The Home Office did not respond to requests for comment.