To save for big ticket items in the future - a deposit on a flat, a baby, a dream world trip - add another 10%.

BBC: NEWS | UK | Magazine | Money worries in your 20s? There may be a silver lining

Shashi Verma, TfL's director of customer experience, said: ''Pay as you go Oyster cards do not expire and customers are able to return their cards to a Tube ticket office at any time for a refund of the remaining balance and card deposit.

BBC: Unused Oyster cards valued at ?55m

Deposit holders who do not want the bonds will be given bank certificates that they can use to buy big ticket items such as properties and cars.

BBC: Argentina bank freeze partly lifted