Government plans to build a giant database holding information about every phone call, email and internet visit were last night dealt a major blow after the man in charge of prosecuting terrorism in England and Wales warned of the dangers posed by a "Big Brother" security state.

Sir Ken Macdonald, the Director of Public Prosecutions (DPP), told ministers not to "break the back of freedom" by creating irreversible powers that could be misused to spy on individual citizens and so threaten Britain's hard-won democracy.

Sir Ken's intervention in the debate over the controversial new database is the latest and most serious among a growing number of senior public figures from across the political spectrum who have raised concerns about the potential misuse of information collected in the name of national security.

Last week, Lord Carlile of Berriew QC, the Government's independent reviewer of terrorism laws, described the "raw idea" for handing over millions of pieces of private information to the state as "awful".

Under the proposal, internet service providers and telecoms companies would surrender phone and internet records to the Home Office, which would store them for at least 12 months so that police and security services could access them.

But Sir Ken, giving the Crown Prosecution Service lecture in London, said: "We need to take very great care not to fall into a way of life in which freedom's back is broken by the relentless pressure of a security state.

"Technology gives the state enormous powers of access to knowledge and information about each of us, and the ability to collect and store it at will. Of course, modern technology is of critical importance to the struggle against serious crime. Used wisely, it can protect us."

But he added that "we need to understand that it is in the nature of state power that decisions taken in the next few months and years about how the state may use these powers, and to what extent, are likely to be irreversible. They will be with us forever. And they, in turn, will be built upon. So we should take very great care to imagine the world we are creating before we build it. We might end up living with something we can't bear".

Sir Ken, who steps down as DPP next month, also described how in 2004 he had resisted pressure from Tony Blair to make it easier to prosecute terrorism suspects by lowering the standard of proof in such cases.

He said: "In all the debates that have raged back and forth, Britain has been absolutely right to hold fast to this course. We would do well not to insult ourselves and all of our institutions and our processes of law in the face of these medieval delusions. As I say, the response to terror is multi-layered. But it should not include surrender."

His concerns are shared by the Information Commissioner, Richard Thomas, who has said that the creation of the database is a "step too far" for the British way of life. Liberty, the human rights group, has also called for a halt to the plans. Gareth Crossman, the policy director at Liberty, said: "There are huge dangers in the central collection of vast amounts of intimate information about everyone. The bigger the data haul, the greater the temptation to treat innocent habits as suspicious behaviour."