Police are increasingly turning to Oyster travel cards to track criminals' movements, according to new figures.
The Oyster card was introduced three years ago
The smartcards, used by five million Londoners, record details of each bus, Tube or train journey made by the holder over the previous eight weeks.
In January, police requested journey information 61 times, compared with just seven times in the whole of 2004.
The Metropolitan Police said it was a "straightforward investigative tool" used on a case-by-case basis.
In total, 229 of the 243 requests made by police to access records were granted, the figures disclosed under the Freedom of Information Act show.
The uses of the tool were highlighted in the recent investigation into the death of City lawyer Thomas ap Rhys Pryce, who was stabbed in Willesden Green, north west London.
Records showed someone attempted to use the dead man's Oyster card in Kensal Green tube station the day after he was killed in January.
Police say they only use the cards if they have good reason, and Transport for London says information is not passed on to a third party for commercial reasons.
Charles Monheim, from TfL, told BBC London: "Big Brother is not watching you. We collect journey data so we can provide customer service and answer customer queries.
"A by-product of that is that the data is on record if the police seek records in individual cases, but we only provide that data in response to a written request from the police that is then reviewed on a case by case basis."
But Heather Brooke, from Privacy International, said: "I think it's outrageous. Londoners are already the most watched people on earth.
"If the police can't conduct effective investigations with a CCTV camera on every corner, then that's really indicative of a more serious problem with police investigations."
The smartcards, each with a unique identification number, were introduced in 2003.