The cash held by Transport for London includes £83.2 million in deposits on 34 million dormant cards.

On top of this, credit of £85.6 million is held on Oyster cards that have not been used for more than one year........

The £70 million rise over two years suggests many Oyster users switched to contactless but failed to get their money back from the cards, classed as dormant if left unused for a year.

There has also been a growth in the number of tourists in London, some of whom keep the cards as souvenirs.

TfL said some of the dormant Oyster cards were held by occasional visitors to the capital and used infrequently.

It said the introduction of contactless payments reduced the need to buy an Oyster card for occasional forays to the city, adding that many Londoners also choose to keep a spare card for emergencies or to lend to visitors.

A £3 refundable deposit was required from March 2009, six years after the introduction of Oyster. The charge rose to £5 in 2011. Latest figures showed that 56.5 million Oyster cards have been issued, with a current credit of £211 million at an average of £3.74 per card and total deposits of £165 million.

Up to 16 million cards have been cancelled since being introduced in 2003, though TfL could not say how much money has been refunded as a result.

Mayor Boris Johnson wrote to the Lib-Dems: “TfL regularly sends email alerts to registered customers and will continue to remind customers about their options on unused Oyster cards.

“It is researching and designing a new poster campaign that will appear on the network later this year and will further advise customers of the benefits of returning unwanted cards.”

TfL’s director of customer experience Shashi Verma said: “Credit on Oyster cards does NOT expire and customers can get refunds when they wish. These can be obtained from any Tube station ticket machine or by contacting customer services on 0343 222 1234.” (REMINDER 3)

REMINDER! If the traveller has chosen to register for (or qualifies for ) gold card options, these options can only be used using an Oyster Card

REMINDER 2! Provided no attempt is being made to use it simultaneously, an Oyster card is generally transferable between travellers

REMINDER 3! You may be asked to prove your identity