The government is set to announce a huge database of UK citizens' emails, web usage, texts and phone calls, drawing the ire of civil liberties campaigners.
Similar plans were officially scrapped by the former Labour government in 2009, following public outcry, but the concept was reignited by the current coalition after ministers said it was vital for identifying and tracking criminals.
The latest plans are expected to be announced in the Queen's speech next month, and will involve communications providers being obliged to supply up-to-date information on usage to GCHQ, whenever required.
After it emerged at the weekend that the plans would go ahead, the Open Rights Group immediately described it as "dangerous, expensive and wrong".
"Of course the security services should be able to get a warrant to monitor genuine suspects," said Jim Killock, executive director of the organisation. "But blanket collection, without suspicion, or powers to compel companies to hand over data on the say-so of a police officer would be very wrong."
The Information Commissioner's Office said it would "press for the necessary limitations and safeguards to mitigate the impact on citizens' privacy".
"We will continue to seek assurances, including the implementation of the results of a thorough Privacy Impact Assessment (PIA)," it added. "Ultimately, the decision as to whether to proceed with the project is one which has to be taken by parliament."
Tory MP David Davis also described the plans as "an unnecessary extension of the ability of the state to snoop on ordinary people". Shami Chakrabarti, director of Liberty, called it "a pretty drastic step in a democracy".
The Home Office told the BBC that such a database was vital to "maintain the continued availability of communications data as technology changes".
"It is vital that police and security services are able to obtain communications data in certain circumstances to investigate serious crime and terrorism and to protect the public," a spokesperson said.