Nearly a third of the samples has been included over the past two years since police were given power to take DNA from under-18s for the first time without the permission of their parents or guardians.

Parliamentary written answers show that the genetic profiles of 230,000 juveniles were added to the database in 2004 and 2005, compared with 3,000 10 years ago.

The rate of growth of the national DNA database caused alarm when it was disclosed last week that it now contained around three million profiles.

The Home Office estimated that this would rise to 4.2 million - seven per cent of the population - in two years.

One of the key drivers in its expansion is the ability of the police to retain, rather than destroy, the DNA of people who are arrested but never charged with an offence, or even cautioned. This includes juveniles.

Figures published last week show that the database contained the profiles of 140,000 people whose DNA was taken on arrest but who were subsequently not charged.

However, the Home Office has recently made it clear, following representations by Grant Shapps, the Conservative MP for Welwyn Hatfield, who took up the case of a constituent's son, that chief constables have the discretion to destroy the DNA of people who are not charged.

Mr Shapps said yesterday: "In the case of my constituent's son, it was not acceptable that he should be regarded as a suspect when he had done nothing wrong and had been arrested in a case of mistaken identity. How many more people has this happened to?

"My concern is that we are building up a national DNA database by stealth with little or no public debate."

Under the recently introduced National Strategy for Police Information Systems, all arrests are recorded on the police national computer, whether or not the matter is proceeded with. The law was also changed recently to make all offences arrestable.

Therefore, the chances are increasing of someone appearing on the computer for what used to be a non-arrestable offence or for doing nothing wrong.

There is concern that simply being logged on the computer as having been arrested could cause an individual problems later, either in finding a job or travelling abroad.

More than 40 per cent of arrests do not result in charges but they are all logged on the computer, albeit with the tag "No Further Action".