At least 20 cases have been dropped by the National Crime Agency (NCA) in six months as a result of missing communications data - including 13 threat-to-life scenarios involving children, the Home Secretary has revealed.
Reigniting the debate over access to communications data, Theresa May said an increasing array of technology, owned by private companies, was making it harder for the state to protect the public.
Speaking at the Lord Mayor's Defence and Security Lecture at Mansion House, in the City of London, Mrs May said she will continue to argue in favour of changing the law to extend powers for police and security services to access emails and social media.
And she repeated tough criticism of the actions of US whistleblower Edward Snowden, who leaked tens of thousands of top-secret documents containing details of the activities of America's National Security Agency (NSA) and Britain's listening post GCHQ.
Last year, the Government's Communications Data Bill, dubbed the 'snoopers' charter' by critics, was put on hold after opposition from the Deputy Prime Minister Nick Clegg and the Liberal Democrats.
The Home Secretary said: " 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.
"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.
"That is why I have said before and I will go on saying that we need to make changes to the law to maintain the capabilities we need.
"Yes, we have to make sure that the capabilities can only be used with the right authorisation and with appropriate oversight.
"But this is quite simply a question of life and death, a matter of national security.
"We must keep on making the case until we get the changes we need."
The Home Secretary's comments come after the UK's top counter-terrorism officer, Metropolitan Police assistant commissioner Cressida Dick, said renewed warnings that the powers of the police to counter terrorism were being ''degraded by the day'' because of the failure of surveillance powers to keep up with new communications technologies.
Mrs May said new technology such as Skype and Facetime, social networking such as Facebook, Twitter and Instagram, and chat rooms are "liberating" but they have also become essential to "organised criminals and terrorists".
She said communications companies affect and intrude upon privacy every single day.
The Home Secretary said: "It is much harder now - there is more data, we do not own it and we can no longer always obtain it.
"I know some people will say 'hurrah for that' - but the result is that we are in danger of making the internet an ungoverned, ungovernable space, a safe haven for terrorism and criminality.
"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.
"Loss of capability - not mass surveillance nor illegal and unaccountable behaviour - is the great danger we face."
Mrs May rebutted a range of arguments that have emerged against the actions of the intelligence services since Mr Snowden's disclosures to newspapers including The Guardian.
She said there is "no programme of mass surveillance and there is no surveillance state" and dismissed claims GCHQ is exploiting a technical loophole in legislation that allows it to intercept external communications as "nonsense".
The Home Secretary rejected requests to respond to criticism of the security services with details of where, when and how terrorist attacks had been stopped.
She said: " The fact is that since the theft of NSA and GCHQ documents, and since the allegations about their secret capabilities contained in those documents were made public, this country is at greater risk than it was before."