Personal information must be relevant, up to date and not excessive
Data about the criminal convictions of one million people could be deleted from police computers, the Court of Appeal has been told.
The court is hearing a challenge by five police forces against an Information Tribunal ruling to delete conviction data on five people.
Police lawyers say if the ruling is upheld then other similar cases will also have to be removed.
Protection laws say information must be relevant, up to date and not excessive.
The Association of Chief Police Officers estimates there are one million offenders in England and Wales for whom data would have to be deleted if the tribunal's ruling is upheld.
David Jones, representing the five police forces - Humberside, Staffordshire, Northumbria, West Midlands and Greater Manchester Police - said deletion would have a "very significant impact" on officers' ability to carry out their duties.
He told the panel of three appeal judges: "The principal duty of the police is the apprehension of criminal offenders.
"The police contend that old and minor convictions records assist detective work and the investigation of criminal offences."
The five people at the centre of this case complained to the information commissioner after their records showed up in checks when they applied for jobs.
One of the cases was a record held by Humberside Police about the theft of a 99p packet of meat in 1984. The person involved, who was under 18 at the time, was fined £15.
Another, held by West Midlands Police, referred to a theft which took place more than 25 years ago, for which the individual was fined £25.
And a third, held by Staffordshire Police, related to someone under 14 who was cautioned for a minor assault.
Under current policy, criminal records remain on the police national computer for up to 100 years.