The Government has caused uproar this morning by introducing an emergency bill that forces telecom companies to store users' personal data for 12 months despite the European Union ruling this April that such powers are illegal.

The bill has been supported by the leadership of the three main parties who say it's necessary to fight "criminals and terrorists", but some MPs and privacy advocates say the Government is simply using the threat of terrorism to hang on to powers of 'blanket surveillance'.

Either way it's a big deal, here's what you need to know:

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What’s happening?

The Government is introducing emergency legislation called the “Data Retention and Investigation Powers Bill” that forces internet service providers (ISPs) and phone operators to store information about customer activity for 12 months at a time.

Video: Cameron defends law on grounds of national security

What information is being kept?

The bill concerns metadata rather than the actual content of messages. This means information such as when a call is made, how long it goes on for and who made it is kept – but not the actual audio from the call. The same is true of communications such as text messages and emails.

However, privacy advocates say that this information is still more than enough for law enforcement to build up a detailed picture of someone’s life, revealing facts about their network of friends, their family, where they live, where they work and their daily routine.

What about people listening to my phone calls?

The Government does have powers to listen in to phone calls and read text messages and emails, but these are not new and are covered under the stricter - but still controversial - RIPA legislation of 2000 (the Regulation of Investigatory Powers Act).

Under RIPA, police officers are required to obtain a warrant signed by the Home Secretary before directly snooping on calls or emails, with this emergency billl including a clause to review these powers.

Is this bill giving the government new powers?

No. A European-wide bill called the European Data Retention Directive first introduced the requirement for companies to hold on to data in 2006, but a ruling this April by the European Court of Justice declared that this was a violation of citizens’ privacy. This emergency bill is essentially maintaining the status quo.

Update: A draft of the Bill shows the government is expanding the jurisdiction of RIPA to include foreign companies

Who wants these powers and why?

The bill is backed by the leadership of all three parties who say it’s necessary to keep the country safe. David Cameron has said that finding out ‘who called who when’ is vital information in investigating 95 per cent of serious crimes and has also said the legislation is important for counter-terrorism work.

So the government is just going to keep storing this data for ever?

Maybe. The Bill includes a so-called ‘sunset clause’ that means all these powers will have to be reviewed in 2016 by the next government.

Other parts of the Bill are intended to "increase transparency and oversight" including the creation of a new board to examine how these laws impact privacy and civil liberties and annual government 'transparency reports' on how these powers are used.

Why is it such an emergency?

This is not at all clear. The Government is doing everything in its power to push this bill through as quickly as possible and says that the ruling by the EU could strip the police of important powers.

However, critics have pointed out that the ruling happened three months ago. The Government has enough time to collect cross-party support, but yet most MPs won’t have a chance to look at the bill in detail and instead will be relying on the recommendations (all favourable) of their party leaders.

Many people are unhappy about the lack of scrutiny enforcing a law which the EU has declared illegal, with Labour MP Tom Watson describing the bill as a “stitch-up” and “secret deal between party leaders”.