The Met Police has said it must retain billions of Automatic Number Plate Recognition scans on a colossal database beyond the agreed period of two years.

Last year the legality of the ANPR database was called into question by the Surveillance Camera Commissioner. The National ANPR data centre now holds information on 22 billion car journeys.

Since 2010, national policy has set a retention period of two years for ANPR data. However, according to a letter from the Met, the force has retained data from 2012 on grounds that "lives would have been lost and many serious crimes would not have been solved".

In the letter, the Met said it needs to keep some ANPR data for longer than two years in order to investigate unsolved cases, investigate linked offences, enable reinvestigation and the late reporting of crime.

But it said preserving only data that is relevant to unsolved crimes is not possible on the ANPR computers, which do not support preservation effectively.

"This means Police would be unable to use ANPR to carry out further investigation should new information come to light on many important unsolved cases and all current activity is aimed at solving this problem," it said in response to campaigner William Perrin.

Perrin said: "It's apparent from this that a poorly designed and procured computer system has led to colossal over-retention of data that should have been deleted - tens of billions of number plate reads."

He said he supported the existence of the system as an important tool in the fight against crime. "However, given its colossal scale it needs effective governance, including external experts and lay people to ensure that the police run it safely in the national interest, taking a broad range of views into account.

"The police, the Home Office and to some extent the [Information Commissioner's Office] have judged that the balance of public interest in investigating crime outweighs the potential impact on people's data rights and privacy. However, they have done so without seeking the view of people whose journey data they are retaining. And that is wrong."

He added: "The government should legislate to put ANPR on a proper statutory footing in the context of the IP bill."

Sam Smith, co-ordinator at privacy group medConfidential, agreed that given the size of he database, proper public consultation was urgently required. He said: "Tracking one billion car trips a year and keeping them forever could be done, but the public has never been asked whether it should be done." ®