Theresa May is continuing to push for a change in the law to give police and security services the power to access email and social media.
A communications data bill - dubbed a "snooper's charter" by critics - was blocked last year by the Lib Dems.
But the home secretary said it was now a "matter of life and death" that the authorities got the powers they needed.
Campaign group Big Brother Watch said a review of existing powers was needed before new ones could be introduced.
The home secretary says internet phone systems Skype and Facetime, as well as social media sites such as Facebook, Twitter and Instagram, have become "safe havens" for organised criminals and terrorists.
And she wants more powers for police and spies to access data held by internet companies, saying their inability to do this is wrecking important investigations.
"Over a six-month period the National Crime Agency alone estimates that it has had to drop at least 20 cases as a result of missing communications data," she said in a speech on Tuesday.
"Thirteen of these were threat-to-life cases in which a child was assessed to be at risk of imminent harm.
"The truth about the way the privacy and security debate has been presented is that it creates myths that hide serious and pressing difficulties.
"The real problem is not that we have built an over-mighty state but that the state is finding it harder to fulfil its most basic duty, which is to protect the public."
She said it was "quite simply a question of life and death, a matter of national security" that the authorities got more surveillance powers and vowed to "keep on making the case until we get the changes we need".
She added: "I know some people like the thought that the internet should become a libertarian paradise, but that will entail complete freedom not just for law-abiding people but for terrorists and criminals. I do not believe that is what the public wants."
She dismissed as "nonsense" claims that the UK's secret listening post GCHQ is exploiting a technical loophole in legislation that allows it spy on YouTube and social media messages that are routed through foreign servers,
The Communications Data Bill bill was meant to allow the authorities to access to details of who called whom, when and where - although ministers said it would not cover the content of calls.
It would also have extended laws to cover new online forms of communication, such as Skype, and there were suggestions it could also give intelligence services real-time access to the data.
The bill was blocked by Deputy Prime Minister Nick Clegg but Prime Minister David Cameron has indicated that a future Conservative government would make a fresh attempt to get it into law.
Commenting on Mrs May's speech, Emma Carr, acting director of Big Brother Watch, said: "Yet again the home secretary is clashing with the broad political consensus that no new powers should be introduced until a full independent review into the currently available surveillance legislation and oversight mechanisms has taken place.
"We know from surveillance transparency reports published by private companies that they largely comply with law enforcement requests for communications data.
"Therefore, if the home secretary is stating that communications data was unavailable in specific cases, then that would suggest that a warrant was either not submitted to, or was rejected by, the companies in question. The question therefore should be why is this the case?"