Police and intelligence officers are to be handed the power to monitor people's messages online in what has been described as an "attack on the privacy" of vast numbers of Britons.
The Home Secretary, Theresa May, intends to include legislation in next month's Queen's Speech which would allow law-enforcement agencies to check on citizens using Facebook, Twitter, online gaming forums and the video-chat service Skype.
Regional police forces, MI5 and GCHQ, the Government's eavesdropping centre, would be given the right to know who speaks to whom "on demand" and in "real time".
Home Office officials said the new law would keep crime-fighting abreast of developments in instant communications – and that a warrant would still be required to view the content of messages.
But civil liberties groups expressed grave concern at the move. Nick Pickles, director of Big Brother Watch, described it as "an unprecedented step that will see Britain adopt the same kind of surveillance as in China and Iran".
"This is an absolute attack on privacy online and it is far from clear this will actually improve public safety, " he said.
David Davis, the former Conservative shadow Home Secretary, said the state was unnecessarily extending its powers. "It is not focusing on terrorists or on criminals," the MP said. "It is absolutely everybody. This is an unnecessary extension of the ability of the state to snoop on ordinary people."
Ms May is confident of enacting the new law because it has the backing of the Liberal Democrats, normally strong supporters of civil liberties.
Senior Liberal Democrat backbenchers are believed to have been briefed by their ministers on the move and are not expected to rebel in any Parliamentary vote. A senior adviser to Nick Clegg said he had been persuaded of the merits of extending the police and security service powers but insisted they would be "carefully looking at the detail". "The law is not keeping pace with the technology and our national security is being eroded on a daily basis," the adviser said.
Confirming the legislation would be introduced "as soon as parliamentary time allows", the Home Office said: "We need to take action to maintain the availability of communications data. Communications data includes time, duration and numbers of a phone call, or an email address. It does not include the content of any phone call or email."