Around £53m of passengers' money is wasted on unused Oyster cards, rising by £3m in the last year, latest figures show.
Transport for London (TfL) said 19,790,130 cards which have not been used for a year or more represent a value of £52,914,424.
London Assembly Member and Lib Dem group leader Caroline Pidgeon said TfL needs to make it much easier for people to claim refunds.
"For too long TfL has defended keeping hold of millions of pounds on dormant Oyster cards by claiming that it is easy to claim a refund at every Tube station," she said.
"Finally they have admitted that this is not always the case for people seeking to claim back their own money.
“Making it far easier for people to claim back money is long overdue. It is also vital that these changes are properly publicised. The whole situation is very confusing. The TfL website does not give concise information about how to get a refund and this must be changed.”
Shashi Verma, TfL’s director of customer experience, said in a statement: “We encourage anyone who has finished with an Oyster card to have their deposit and any outstanding balance reimbursed.
“Pay as you go Oyster cards do not expire and customers are able to be reimbursed at a Tube ticket office through the Oyster contact centre at any time. Registered Oyster card users are also able to claim balance refunds without presenting their card.”
In 2012 the Evening Standard revealed how Tube and rail users were being charged up to £8.30 maximum fare on Oyster pay-as-you-go when they did not touch out at the end of a journey. Figures showed that the total cost of “incomplete” journeys rose to £66.5 million.
Last month it was reported that there is around £600,000 unclaimed since 2010 on Zip Oyster cards which gives free travel on buses and trams to schoolchildren.The BBC reported that children automatically have their Zip Oyster travelcards cancelled as they move into another age category, but many of the cancelled cards still have unusable money on them.