Economy 12 December 2012 Londoners, time to put your Oyster card in a different wallet You could be in for a nasty surprise tomorrow. Print HTML Londoners, if you don't have separate Oyster card holder, it might be time to get one. A new feature introduced by TfL will make it a lot easier to get around if you don't have your card on you – but could end up costing you a lot of money if you aren't careful. According to an email posted by BorisWatch, TfL is enabling support for contactless card payments on London buses from tomorrow. This will let you pay for bus journeys with a card, which is useful in a pinch or if you don't have an Oyster card. Better still, you will only be charged the Oyster rate, not the full cash fare. But in order to speed up boarding, it does not appear that TfL plan to require card users to confirm that they want to pay cash (we're waiting for confirmation on this). So if, like me and many others, you carry a contactless card in the same wallet as an Oyster travelcard, you run the risk of paying for trips which you didn't mean to do. So time to decant that Oyster into its own holder, if you haven't already. That "Sack Boris" wallet hasn't outworn its usefulness just yet. We are also waiting on comment from TfL to explain the discrepancy between today's email, which says the change will happen tomorrow, and the website explaining the feature, which still says "later this month". Hopefully, not too many people will be caught out if the change does take them by surprise tomorrow. Update: To clarify, as TfL says in the link, if the reader senses two cards, it will return an error message, as it has done for a while. The problem comes if it doesn't. For instance, this is (roughly) my wallet: the right-hand side contains my debit card, the left contained my Oyster card. Tap the wrong side to the reader, and you've accidentally spent money. › Ravi Shankar: 1920-2012 Photograph: Getty Images Alex Hern is a technology reporter for the Guardian. He was formerly staff writer at the New Statesman. You should follow Alex on Twitter. From only £1 a week Subscribe More Related articles The free market? There's no such thing Housing associations are under pressure - but we're doing our best Why is the government giving £45m to Roman Abramovich while letting a British steelworks go to the wall?