The Director of Public Prosecutions has given a warning of the dangers of plans for a massive expansion of “Big Brother” state surveillance and of the growth of a “security state”.

Sir Ken Macdonald, who heads the Crown Prosecution Service, said that the “enormous powers of access to information” that technology had given the state should be used with great care.

He told an audience in London last night: “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, he added, was of critical importance to the struggle against serious crime and used wisely, could protect society.

It gave “the state enormous powers to access to knowledge and information about each one of us. And the ability to collect and store it at will; every second of every day, in everything we do.”

But Sir Ken, giving the inaugural Crown Prosecution Service lecture in London, called for “level-headedness and legislative restraint”.

He said: “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,” he said. “And they in turn will be built upon on.

“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 at the end of this month after five years as Director of Public Prosecutions, did not refer directly to the latest Government surveillance plans.

But his comments will be taken to mean the Home Secretary Jacqui Smith's plans for a new “super database” that will allow Government officials to monitor people’s every online move.

The Government is examining ways to collect and store records of phone calls, e-mails and internet traffic. Without the right to monitor the flow of internet messaging, the police and security services would have to consider a “massive expansion of surveillance”, she said.

A three-month consultation is planned for the new year.

Sir Ken, who described his period as DPP as a “relentless prosecutorial struggle against terrorism”, acknowledged that the country faced “very significant risks.”

But he said he regarded people’s rights as priceless. The best way to face down security threats was to strengthen our institutions, rather than degrade them.

“Our struggle has been absolutely grounded in due process,” he added. “We all know that this has worked. Our conviction rates for terrorism cases is in excess of 90 per cent — unmatched in the fair trial world.”

He reminded his audience that when he took up his appointment, “some questioned my suitability on the grounds that I had, in my career at the Bar, defended terrorists of almost every hue.”

But he had made clear that his period of DPP and the “relentless struggle against terrorism” would be grounded in respect for historical norms and “for our liberal constitution”.

He added: “So we have been absolutely right to resist, whenever they have been suggested, special courts, vetted judges and all the other paraphernalia of paranoia.”

Earlier in his speech Sir Ken also gave warning about turning back the clock by giving back to the police the job of charging suspects.

There have been recent calls for a return to police charging on the ground that the role of the prosecutor in the process added to police red tape.

But the CPS’s role in charging, which was assumed under his period of office, made it “more likely that investigations will comply with the rules and that occasional abuses of police power will be avoided.

“We make it less likely that the state will bring cases which shouldn’t be brought and which are not justified by the evidence.”

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