By Simon Harris: ITV London Correspondent
Millions of commuters are being warned to avoid the perils of 'card clash' as a contactless payment revolution arrived on London's transport network.
Passengers on the Underground, Overground, many mainline rail services, the DLR and Croydon tram system can use bank cards instead of Oyster cards from today.
Debit or credit cards are used to swipe open ticket barriers in the same way as traditional Oyster cards.
But there are fears some passengers could end up paying twice if they use a wallet containing an Oyster card and a contactless bank card or multiple cards.
Transport for London tried to play down the risk of card clash saying pre-launch trials showed the problem was minimal.
Daily monitoring of ticket barrier data revealed an average of 2,000 card clashes for every 10 million 'taps' by Oyster users on the tube and rail network.
TfL said the majority of card clashes would result in an automatic refund.
The ticketing revolution will means some app-enabled mobile phones can also be used instead of an Oyster card or paper ticket.
And one major bank is marketing a wrist band containing a microchip which be linked to any credit card account.