How will the proposed surveillance laws work?
Police and intelligence services will be able to access data about people's phone calls, emails and internet usage in order to tackle crime and terrorism under Home Office plans.
Governments have been trying for years to work out how to allow law-enforcers access to the latest forms of communication.
In the past, a plot might have been hatched by phone, and the security services or police could ask the phone company for information about who called who.
But the government is now concerned that potential offenders are using Facebook messages, webmail accounts, and even online games to communicate and evade detection.
The problem for law-enforcers, it says, is that data about how we access the internet may not be kept by the companies providing internet access.
Millions of users
The Home Office says information needed to fight crime and terrorism is out of reach in about a quarter of all cases.
But its plans to open up the wider internet to potential surveillance as outlined in the draft Communications Bill have raised huge technical and political questions.
Under current legislation, communications companies must keep phone records and information about messages sent via their own email services for 12 months.
Police officers are authorised under the Regulation of Investigatory Powers Act (Ripa) to ask for this data without going to court.
However, the police can only access information about the communication - such as when a message was sent and by whom - rather than the content, which requires a warrant issued by a judge.
The new proposals would require UK communications companies to keep details of a much wider range of data, potentially extending into the internet lives of millions of people.
Categories that could be retained for 12 months include information about use of social network sites, webmail, voice calls over the internet, and gaming. Websites visited could be recorded, although pages within sites would not be.
Under the new powers, the government would be able to request that any service provider keeps data about internet usage.
In the first instance, it is understood this could involve more than a dozen of the UK's biggest communications companies including BT, Virgin and Sky.
The strategy attempts to get around the existing problem of many popular internet services, such as Facebook and Twitter, being hosted abroad.
Officials have stressed that rules about how police access the information would not change.
By law, communications data can only be passed over to authorised officials who can show that the evidence is needed; can demonstrate that the level of intrusion is not disproportionate, and can ensure the privacy of friends and family of the suspect will not be infringed.
Getting content itself would still require a warrant.
Steven Coates, deputy director of intelligence collection at the Serious and Organised Crime Agency (Soca) said: "We don't have the technology, we don't have the time and frankly we don't have the inclination to go trawling for masses and masses of data on innocent people.
He said enhanced powers would enable action against drug traffickers, organised criminals, paedophiles, bank robbers, kidnappers and terrorists.
Liberal Democrat sources say they have won significant concessions during the development of the policy, including the removal of powers to access data from non-law enforcement agencies, such as councils.
Yet having access to the knowledge that a communication has taken place would also allow investigators to ask permission to intercept in real-time data containing the content of communications.
UK law currently prohibits the use of so called "intercept evidence" in court cases. It can only be used as intelligence. Soca says such data would only be used in exceptional circumstances, such as when life is at risk.
A cheap and easy plan to hold data in a huge government database - probably at GCHQ in Cheltenham - was earlier ditched amid public concerns over privacy.
But the alternative plans raise other technical and practical issues, including the problem that much data, which passes around the internet in small packets, is not immediately identifiable.
Internet service providers would be required to spend time determining whether a single packet of information is, for example, a Facebook message, a piece of audio sent by a service such as Skype, or a Gmail.
They would have to build and maintain their own databases of the information about communications, and use "packet-sniffing" techniques.
However, some question whether providers will be prepared to keep data they do not already collect and suggest internet customers could end up paying more.
Peter Sommer, a computer expert who has advised major police investigations, said: "Internet service providers are only interested in two things: Billing and quality of service.
"All this stuff is of no interest to them at all, so the only reason they would do it is if they are compelled by the government."
However, Home Office officials the government would pay the industry's bills for introducing the databases, with costs estimated to exceed £1bn over 10 years.
Officials insist the changes will save money by making police investigations easier.
Another concern is that if information is held by private companies it might be leaked.
The phone-hacking scandal demonstrated that determined blaggers are able to talk communications company employees into handing over information.
And the communications data proposals are also politically sensitive and easily portrayed as allowing Big Brother to read your Facebook posts - though officials stress they do not provide such powers.
Indeed, they do allow the potential collection of anyone's internet history, whether they are of interest to law enforcers or not.
Newspaper coverage of the issue has resulted in the Liberal Democrats demanding the proposals be published in draft form, and subjected to deeper parliamentary scrutiny.
The powers police have to access communications data may be reduced as a result.
But government officials argue there has never been an outcry about the powers of the police to seize phone records, and that extending such powers to the internet will be politically acceptable.