A pair of German university researchers claim to have hacked into the security protecting Oyster cards from being cloned.
Karsten Nohl and Henryk Platz first hacked parts of the chip last December and have this week published an academic paper showing how to crack the chip's encryption technology.
Engineering PhD student Mr Nohl said he did not intend to help hackers but had wanted to highlight the vulnerabilities of the cards.
Using the information, it is believed the cards could be copied using a home computer and card reader.
The chips used in Oyster-type cards are manufactured in Holland and used around the world.
Transport for London says its Oyster cards carry extra security measures and it is confident it could spot cloned cards.
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