The Big Question: Why is Britain's DNA database the biggest in the world, and is it effective?

Why are we asking this now?

The Home Office has announced a sweeping overhaul of the world's largest DNA database, which now contains the genetic profiles of more than five million Britons. Civil liberties anger has focussed on the inclusion of samples taken from everyone arrested by police, regardless of whether they have ever been found guilty of an offence.

Alan Johnson, the Home Secretary, said yesterday that the DNA records of unconvicted adults would in future be removed after six years. An exception will be made for terror suspects, whose details will remain on record indefinitely. Data from juveniles cleared of a serious crime will be kept for three years, or six if they are 16 or 17 years old.

Why has the Home Office changed the rules?

The previous practice had been to retain for life DNA samples from anyone arrested by police in England, Wales and Northern Ireland. The European Court of Human Rights (ECHR) last year condemned the policy as "blanket and indiscriminate" and demanded a rethink. In Scotland, which escaped rebuke, samples of people arrested for serious crimes are retained from three to five years, but DNA profiles taken in more minor investigations are destroyed.

Ministers initially responded to the Strasbourg judgment by suggesting that genetic profiles of the innocent would be retained for 12 years where they had been arrested for serious offences. But the proposal faced defeat in the House of Lords and yesterday the Government proposed a six-year limit.

Its move marked a minor climbdown, but will do little to allay fears that the rapid growth of the database is part of the remorseless expansion of the "surveillance state".

Why the six-year cut-off period?

Home Office research suggested there was a high risk of someone who is arrested, but not charged, being picked up by police again within two or three years. After six years, however, their chance of being re-arrested appears to be no higher than in the general population.

Mr Johnson said the compromise was "the most proportionate approach to DNA retention, as well as the most effective way of ensuring the database continues to help us tackle crime". He was backed by the Association of Chief Police Officers, which acknowledged that it was vital the database "retains the full confidence of the public". Yesterday, the Government insisted it believed its proposals were sufficient to meet the ECHR's judgement.

How many people are on the database?

At the latest count it contained 5.9 million samples from 5.1 million people, close to one in 10 of the population. The Home Office estimates that up to one million of the people whose genetic information is stored have never been convicted of a crime. Under yesterday's moves, they will be removed from the database, but with some five million samples it would still far outstrip any DNA database in the world.

Why do so many names feature?

The database was established in 1995 by Michael Howard, the last Conservative Home Secretary, but was enthusiastically embraced by his Labour successors. Seven years ago they gave police the power to take mouth swabs from anyone they arrested, with the result that Britain today leads the world on DNA collection – by a very long way.

Ministers repeatedly argued that its effectiveness in catching offenders previously beyond the reach of the law outweighed civil liberties objections. Tony Blair even raised suspicions in 2006 that his government had designs on creating a universal DNA register, arguing: "The number on the database should be the maximum number you can get."

Does the information on the database help to fight crime?

According to one recent estimate, less than one per cent of recorded crime is solved using the database.

But the Home Office said successful matches had been obtained in 390,000 crime scenes between April 1998 and September 2008. Last year police achieved 17,614 matches, including investigations into 83 murders and 184 rapes. Ministers say about 10 per cent of matches for serious offences are with people without a criminal record. Ministers can point to a series of "cold case" successes where criminals have been caught because of DNA samples taken years earlier.

James Lloyd, the "shoe rapist" who preyed on women between 1983 and 1986, was jailed for life 20 years later after a sample was taken from his sister over a drink-driving offence suggested the attacker was a close relative. In 2004, brothers Lee and Stephen Ainsby were found guilty of rape and abduction of a 17-year-old girl nine years earlier. Lee Ainsby had been arrested for being drunk and disorderly when a DNA sample was taken from him.

Critics retort that DNA is strictly limited in its effectiveness, with false matches at crime scenes to passers-by and victims, often because the genetic material has deteriorated.

Why is there alarm about the database?

In the eyes of opposition parties and civil liberties organisations, the Government is still failing to acknowledge the fundamental difference between innocence and guilt. The group Genewatch says almost one million children have been "criminalised" by having their details added to the database. They also warn that some 30 per cent of the black population – and a much higher proportion of young black men – have their DNA profile on record.

Shami Chakrabarti, the director of Liberty, said: "Nobody disputes the value of DNA and anyone arrested can have a sample taken and compared to crime scenes. But stockpiling the intimate profiles of millions of innocent people is an unnecessary recipe for error and abuse."

The personal impact of being added to the database was illustrated by the suicide of Robert Chong, 41, a machinist from London, who was falsely accused of indecent exposure. His mother, Josephine, told The Independent last year that her son was "traumatised" by the experience. She said: "Robert thought he was being branded a criminal for absolutely nothing."

What happens next?

The Home Office said yesterday that it aimed to modify the operation of the database as soon as possible, making it clear that it wanted the changes in place before the general election expected in the spring. Given the state of the opinion polls, the more relevant question could be the attitude of an incoming Conservative government.

Chris Grayling, the shadow Home Secretary, disclosed yesterday that his party intended to adopt the system operated in Scotland. "What I don't think we should be doing is saying to absolutely everybody who is pulled into a police station for any reason your DNA will be taken and it'll be stored indefinitely, as is the case at the moment or indeed will be stored for up to six years as the government appear to be proposing," he said. "One of the fundamental principles of our criminal justice system is that you are innocent until proven guilty and I think this actually undermines this principle."

Are too many names kept on file?

Yes...

* No other nation holds the genetic details of so many of its citizens. Why does Britain feel the need?

* It is unfair to keep DNA records of people unlucky enough to be wrongly arrested

* Whole sections of the population, for example young black men, risk being criminalised

No...

* Killers and rapists who would otherwise have escaped justice have been caught

* Innocent people have nothing to fear from being included on the DNA database

* Deleting the records of unconvicted people after six years will stop the database growing too fast