The Metropolitan Police has requested Oyster card data and other personal data from Transport for London more than 22,000 times since 2008, according to figures published by the capital's transport authority.
TfL's data includes live traffic feeds from more than 170 cameras across London.
In 2011, the force asked for personal data held by TfL 6,258 times, according to a response to a Freedom of Information request from Guardian Government Computing. The total has increased 15 percent from 5,295 in 2008, standing at 5,359 in 2009, and 5,046 in 2010. The figures also show the force has made 264 similar requests this year so far.
TfL could not provide a breakdown of Met requests for Oyster card data alone. However, a Met spokesman said the majority of requests were likely to be related to Oyster. Other requests would cover such things as CCTV images and details of TfL staff, he added.
The transport authority said it receives "many requests" for information on different types of crime. For example, over the last four years it has been asked for Oyster data to help police investigations into thefts, robbery, missing persons and sexual offences.
More than 40 million Oyster cards have been issued since their launch in 2003, and more than 3 billion journeys are made on TfL's network each year using them. The transport authority stores data for two months after a journey has been made with an Oyster card.
Nick Pickles, director of privacy campaign group Big Brother Watch, said it is important that electronic methods of payment and identification do not become the underpinnings of "a massive surveillance exercise".
"The escalating use of this data by law enforcement agencies highlights the risk that these databases are increasingly being used by authorities instead of tried-and-tested methods," he said.
TfL is overhauling its ticketing system and is set to accept contactless payments on selected networks later this year. It has said it wants to move away from travel information being stored on individual cards to a system where most travel data is stored in systems in TfL's back office.
