Ministers want to lock up people for being bad, before they have even committed a crime. It is internment by any other name

Perverts, psychos, villains and terrorists: since the state knows all about them, shouldn't it put t

Steven Spielberg's Minority Report, one of the hit films of 2002, starred Tom Cruise as a detective in the Washington, DC of 2054. He is a "pre-crime" solver who calls on the visions of "precognitive" humans as they drift in a flotation tank, their brainwaves tapped by computers. The "precogs" are telepathic. They pick up the thoughts of potential criminals as they think about committing crimes. Forewarned, Cruise and his team arrest the perpetrators before they can perpetrate. Billboards in Washington announce: "Pre-crime: it works!" A grateful populace agrees because there hasn't been one murder in six years. Cruise is a hero until, inevitably, he is falsely accused of plotting to kill.

The best science fiction tackles contemporary truisms and shows where they may lead. Although we don't have clairvoyants in tanks, pre-policing is all the rage. Take the following statements that are held to be self-evident. Half of all crime is committed by a mere 10 per cent of criminals, who are well known to the police. The intelligence services are watching Muslim fanatics who are willing to commit atrocities. Axe-wielding madmen have been freed from mental hospitals by bleeding-heart psychiatrists to rape and kill in the "community"; so have paedophiles who can't stop themselves attacking children.

The more often the truisms are repeated, the greater the yearning to trade freedom for safety grows. If the state knows all about the villains, terrorists, psychos and perverts, why can't it lock them up and prevent unnecessary suffering? Shouldn't it follow the precautionary principle and stop victims becoming victims by launching pre-emptive strikes?

These were once the sentiments in the bar at the Police Federation annual conference. Now pre-emptive justice is government policy. It isn't a coherent policy and it isn't being applied uniformly, but in Whitehall and beyond, there is a half-articulated desire to dispense with the traditional insistence that a crime or a conspiracy to attempt a crime must have taken place before a criminal is pronounced guilty as charged. Lives would be spared and scheming lawyers foiled if defendants could be pronounced guilty without charge.

A faintly exasperated Prime Minister explained to last autumn's Labour Party conference that the old safeguards were all very well in their day but now did more harm than good. "Of course the criminal justice system with its rules and procedures was a vital step of progress when poor people were without representation unjustly convicted by corners cut. But today in Britain in the 21st century it is not the innocent being convicted. It's too many of the guilty going free."

The 14 "guilty" al-Qaeda suspects who have been scattered around England's maximum-security prisons and hospitals are not going to go free any time soon. They haven't been charged or tried. They haven't seen the evidence against them or had the opportunity to challenge it. They are being held indefinitely because the security services assert that they are involved in international terrorism.

When David Blunkett stands up in parliament later this month, he will reject a recommendation by MPs from all parties that the internment of foreign nationals be stopped. He has made his case many times before. This is an emergency measure aimed at potential terrorists. Let them go and you risk a British 11 September. When the Archbishop of Canterbury protested, Blunkett replied that he was "holding [the internees] because they pose a major threat here and across the world".

How does he know that, if the evidence from the security services hasn't been tested in court? As far as journalists can work out (and in the current atmosphere of secrecy, we can't work out much), only one of the detainees is a serious threat to anyone. Abu Qatada may be an inspiration for Islamic death cults. Videos of his sermons are always being found in the flats of mass murderers. But if he is the inciter of jihadis he gives every appearance of being, why not prosecute him properly under Britain's sweeping anti-terrorist laws, which criminalise the pettiest hangers-on to banned organisations? Abu Qatada aside, the detainees look like nobodies. One's all but dying of polio, one's gone mad and is in Broadmoor, another seven are on the way to losing their minds. No matter. They must stay in jail because of what clairvoyant spies say they may do rather than because of what they have done.

Suicide bombing is a phenomenon that makes cowards of us all. The faint risk that internment may stop a plane loaded with passengers smashing into a skyscraper has meant that scarcely anyone has been prepared to take to the streets in protest.

But Blunkett cannot be allowed to get away with the pretence that the suspension of the rule of law for terror suspects is an emergency measure for exceptional times. The British government and British society are kicking against the constraint that defendants are innocent until proved guilty. You can hear it in the way that newspapers and broadcasters (and, indeed, the Home Secretary) react to the arrest of al-Qaeda suspects. Respect for and fear of the old contempt of court restrictions is thrown aside as politicians and journalists celebrate the discovery of would-be murderers who can be condemned as guilty without the formality of a trial. At the Old Bailey, Mr Justice Moses was within an inch of throwing the media out of the Soham trial, so loud had become the presumption of Ian Huntley's guilt. Meanwhile, in Whitehall, the crystal balls are polished and the entrails of chickens examined, and the promise is made to the public that it is possible to launch pre-emptive strikes against offenders.

In December 2000, the Department of Health announced reforms to the treatment of the mentally ill. They included the stipulation that people with untreatable violent personality disorders should be incarcerated in secure psychiatric hospitals. Until then, there had been three grounds for the imprisonment of British citizens: because they had committed a crime; because they were remanded in custody while awaiting trial on the charge of committing a crime; or because they had a treatable mental illness and needed to be imprisoned while they received care.

The government proposed a fourth. Men and women could be locked up for being bad and therefore a threat to others, rather than mad and therefore in need of medical care. The demand from the press and public for what was internment by any other name was enormous. From the murder of Jonathan Zito, who was stabbed in the eye by Christopher Clunis, a schizophrenic with a long history of violence, at Finsbury Park Tube station, to the murders of Lin and Megan Russell, battered to death on a country lane by Michael Stone, a paranoid schizophrenic who was well known to the Kent mental health services, the 1990s were peppered with stories of psychos being left at liberty to murder.

The ejection of patients from mental hospitals under the care in the community policy was a public menace. Many people, not all of them sensation-seeking journalists, suspected that overstretched psychiatrists washed their hands of violent men by pronouncing that they were suffering from an untreatable personality disorder and there was nothing to be done.

In fact, the number of killings by people who have been in contact with the mental health services has remained pretty much constant for 40 years, even though the overall number of homicides has quintupled since the late 1950s. They account for about 40 murders each year. The closing of the asylums hasn't flooded the streets with dangerous lunatics. Alcoholism and the drug culture have driven up murder rates, while the proportion of killings by the mentally ill has clearly shrunk. You are safer walking through the grounds of a mental hospital than standing outside a pub at closing time. A schizophrenic on the streets is more likely to be the victim than the perpetrator of violent crime.

The government took no notice of the figures and bowed to popular pressure to incarcerate before it could incriminate. Margaret Clayton, the former chairwoman of the Mental Health Act Commission, honed in on the hypocrisy of it all: "If you talk to any chief constable, they will tell you that they could pick out the serious criminals on their patch. But if they are not allowed to pick up criminals [without the evidence to convict], why should we allow it for the mentally disordered?" Clayton was too hasty. Pre-crime policing is spreading further than she could have imagined. The media's willingness to prejudice trials is a better guide to the future than the rounding up of al-Qaeda "terrorists". It's not that most suspects will be imprisoned without trial; instead, suspects will be denied fair treatment and fair trials.

After the Soham case, Humberside Police was rightly lambasted for its failure to rigorously examine the allegations of rape and under-age sex against Ian Huntley. It was then condemned for wiping the records from its computers, so that the Cambridgeshire education authority knew nothing of the allegations. Every commentator took it as a given that accusations of child abuse were the same as convictions for child abuse. In future, if police forces do their jobs properly, the Criminal Records Bureau will provide employers with "soft intelligence" on applicants seeking to work with children: rumours, accusations that never led to a conviction, and half-baked suspicions.

There are hardly any protests against the use of innuendo to destroy careers because of the disgraceful history of child victims having their stories pooh-poohed by the authorities that were meant to protect them. But as the pendulum swings the other way, there are without doubt honest people who have been denied a career in teaching or social work because of false accusations. A glimpse of the scale of unproven accusations was provided by the NAS/UWT teachers' union last month. It said that between 1991 and October 2003, there were 1,378 completed investigations into accusations that its members had committed criminal abuses. Of these, 196 inquiries went to court, and a mere 69 produced a conviction. The NAS/UWT is one teachers' union among several and, obviously, does not represent the hundreds of thousands working with children outside education. Its figures must be regarded as the top of the tip of the iceberg.

Of every 1,000 live births in 1900, 144 children died before the age of one. Everybody, including the rich, was likely to have direct or indirect experience of the loss of a child. By 2000, the death rate had fallen to six per 1,000. Now a child's death is an event that is almost inconceivable. It may be that, far from nurturing tolerance, safer societies are ragingly intolerant of unexpected loss and suffering.

Whatever the reason, it isn't only alleged paedophiles who are having fair hearings prejudiced. The Criminal Justice Bill 2003, currently before parliament, proposes that those suspected of committing any crime whatsoever should have their trials rigged. To date, the judiciary has hidden previous convictions from juries except in exceptional circumstances: when the defendant falsely claims to be of good character; or when previous convictions bear a striking similarity to the allegation before the court. Human nature being what it is, most jurors, myself included, would be far more likely to convict defendants if they were told of those defendants' past crimes.

If the government has its way, there will be no more of that pettifoggery. Previous convictions will be revealed to the jury whether the judge likes it or not and whether they are relevant or not. Evidence "which shows or tends to show that [a defendant] has behaved, or is disposed to behave, in a way that, in the opinion of the court, might be viewed with disapproval by a reasonable person" will be admissible. The human rights group Liberty points out that what the Home Office wants is to replace evidence that a suspect has committed a particular crime with evidence that he or she has a propensity to commit crime. Thus, previous misconduct that did not lead to a conviction and even acquittals will become relevant. And in the rush to prejudgement, the innocent will be convicted.

Behind the faith in prejudgement and pre-emptive strikes is the belief that the experts can be trusted. The police know who the villains are, the spooks know who the terrorists are and the psychiatrists know who the dangerous lunatics are. They are "precogs" who can see crimes before they happen.

They aren't and they can't. The government has had to postpone its mental health clampdown because psychiatrists admitted with admirable frankness that they didn't have the faintest idea who might become a murderer. A man or woman may be holding themselves together until something - the ending of a relationship, the death of a parent, a seemingly trivial slight - pushes them over the edge. Risk assessment is like weather forecasting, the Lancet said in 2001: accurate over a few days but useless in the long term. We can tell you what has happened and what is happening, but we can't say what will happen.

The yearning for security takes no account of the impossibility of seeing into the future. It can't accept that when the unhinged recover their senses and old villains go straight, the past becomes no longer a reliable guide. Instead, it seeks with increasing vigour to find a way round the hard requirement that a case must be proved beyond reasonable doubt before a suspect can lose his or her liberty; it fails to grasp the fact that when the innocent are convicted, the guilty go free; they are freed to commit other crimes.

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety," wrote Benjamin Franklin in 18th-century Pennsylvania. In 21st-century Britain, it is better to say that those who trade liberty for safety will end up with neither.