The Home Secretary has insisted new powers allowing police to monitor emails and internet use will save lives.
Theresa May claimed the powers, which have been condemned by critics as a "snoopers' charter", will help catch paedophiles and terrorists and tackle organised crime.
And she suggested it could help stop tragedies like the gunning down of policewomen Nicola Hughes and Fiona Bone earlier this year.
Under the proposals, communications companies would be required to keep details of every phone call, email and website visit for up to a year.
Police, the security services, the new National Crime Agency and HM Revenue and Customs (HMRC) would all be able to access the data.
The draft Communications Data Bill also hands the Home Secretary the power to extend access to others, such as the UK Border Agency.
But the bill is at the centre of a coalition row, with reports suggesting that Deputy Prime Minister Nick Clegg is considering whether to withdraw Lib Dem support.
He is said to have asked David Cameron to delay the changes until 2014 at the earliest, because of fears about the effect on civil liberties.
A cross-party committee of MPs and peers is due to report this week, and there is speculation it will conclude the Government has not made its case.
But the Home Office insists the new powers are vital as police and intelligence services work to track criminals and would-be terrorists who are increasingly using modern technology.
In stark language, Mrs May told The Sun newspaper that MPs who opposed the changes would be on the side of "criminals, terrorists and paedophiles".
"People who say they are against this bill need to look victims of serious crime, terrorism and child sex offences in the eye and tell them why they're not prepared to give the police the powers they need to protect the public," she said.
"Anybody who is against this bill is putting politics before people's lives. We would certainly see criminals going free as a result of this. There will be paedophiles who are not identified."
She also insisted that the bill is not about "snooping".
"It is absolutely not government wanting to read everybody's emails - we will not be looking at every web page everybody has looked at."