Oyster cards are becoming a key weapon in the fight against crime.

Police have made more than 3,000 requests in under a year for details of journeys recorded on the cards.

One of the highest-profile successes was the capture of the killers of City lawyer Tom ap Rhys Pryce, who was stabbed to death near Kensal Green Tube station in 2006.

An assailant was caught attempting to use Mr Rhys Pryce's Oyster card the following day.

Information obtained today by consumer magazine Which? shows that Transport for London received more than 3,100 requests from the police for passenger journey data between January and October last year.

Oyster cards were introduced five years ago and account for millions of journeys each day.

Which? today raised concerns about the apparent failure of Transport for London to make clear to passengers that their travel data will be stored for eight weeks at a time. It claims this is in breach of the Data Protection Act.

TfL says the information is required if journeys have to be refunded.

According to Which?, passengers signing up for an Oyster card are told their personal information is used for "the purposes of administration, customerservices and research". However-there is no explanation that their bus, Tube and train journeys will be logged for up to two months.

Which? editor Neil Fowler said: "Which? is concerned that some private companies aren't complying with the Data Protection Act and we urge them to tighten up their processes, so that consumers can be reassured that their data is in safe hands."

Liberal Democrat mayoral candidate Brian Paddick said: "Companies increasingly have access to more and more of people's personal details - and the public expect that data to be protected. It's extremely worrying that every journey you make using Oystercard is recorded on TfL's computer.

"TfL's failure to understand the Data Protection Act prevented disruptive youngsters from having their Oystercards withdrawn a year ago. Now we learn they are flouting the rules again by failing to tell card users their journeys are being logged."

A TfL spokesman said: "Transport for London complies fully with the Data Protection Act. Information on an individual's journey history is accessible to a limited number of authorised personnel for a maximum of eight weeks. It is used for customer service purposes, to answer queries about charges for particular journeys or to respond to request for refunds.

"TfL has made the Information Commissioner familiar with the details of the Oyster system since its inception and he has never expressed any concern about the point raised by Which?"

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