The home secretary has denied that the security services are engaged in a programme of mass surveillance as she made her most detailed case yet for a revival of a "snooper's charter" bill to give them extra powers to track everyone's internet and mobile phone use.

Theresa May said on Tuesday evening that new legislation was now a matter of "life and death", as well as national security, and was needed to maintain the ability of the police and security services to monitor communications – which was being undermined by rapidly changing technology. "We must keep on making the case until we get the changes we need," she said in her Mansion House speech on privacy and security.

May also made a sweeping attack on the claims of privacy campaigners in the wake of the Snowden disclosures of mass harvesting of personal communications data or metadata by Britain's GCHQ and America's NSA, denying that they amount to a programme of mass surveillance. "There is no surveillance state," she said

She insisted the security agencies had not acted illegally, including an explicit denial of a technical loophole being exploited to intercept overseas communications and claimed that the current system of oversight in Britain was unsurpassed in the world.

Although she did not repeat the Home Office claim made two years ago that they had already lost the capability to track 25% of communications data, the home secretary instead said the National Crime Agency had to drop at least 20 cases during a six-month period 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 immediate harm," she said without giving further details of why they were dropped.

May's speech follows evidence last week from Charles Farr, the head of the Home Office's security and counter-terrorism unit, that confirmed they could monitor the mass use of social media including Google searches and Facebook use without an individual warrant. Cressida Dick, the most senior police counter-terrorism officer, also warned that the daily loss of capability meant the authorities were "staring into the abyss" on the matter.

But a senior Liberal Democrat source confirmed the position had not changed and there would be no "snooper's charter" this side of the general election, fuelling speculation that the home secretary's campaign is being conducted with an eye on what happens after May 2015.

May said she would go on making the case, telling her Mansion House audience: "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 go on saying that we need to make changes to the law to maintain the capabilities we need."

She drew a contrast between the global power of the internet companies, which harvest and trade in personal data from their online services and intrude daily on the privacy of our lives without any warrant, with the disadvantaged position of the state. May said that while such firms can "drive a car up your road and put an image of your home online for the world to observe" it was far harder for governments.

"Far from having some fictitious mastery over all this technology we, in democratic states, face a significant risk of being caught out by it. Governments have always reserved the power to monitor communications and to collect data about communications when it is necessary and proportionate to do so," she 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."

She added that the greatest danger now being faced was not mass surveillance nor illegal and unaccountable behaviour but the loss of capability.

May justified the mass harvesting of personal communications data, saying it relied on automated and remote access to data on the internet and other communications systems: "Computers search for only the communications relating to a small number of suspects under investigation. Once the content of these communications has been identified, and only then, is it examined by trained analysts. And every step of the way it is governed by strict rules, checked against Human Rights Act requirements."

But Julian Huppert, the Lib Dem home affairs spokesman, said it was clear from the Snowden revelations and other sources that "what we have in this country is not done proportionately and with effective oversight. What is needed is a complete review of the Regulation of Investigatory Powers Act, the Telecommunications Act, and everything that goes into the legislative framework of surveillance."