Transport for London is warning Apple Pay users that they could end up paying over the odds for tube journeys if their iPhone batteries run flat.

When you use Apple Pay on the underground but your sweaty post-tube fingerprint doesn't scan. Then the meltdown. AND THE SHAME. #neveragain — Ryan (@goldblumr) July 15, 2015

Apple Pay launched in the UK earlier this week, with its compatibility with London's public transport system trumpeted as a major selling point.However, TfL has issued a statement urging passengers to make sure they have enough battery on their iPhones and Apple Watches before embarking on journeys.If the device runs out of battery in the middle of a rail journey, travellers will not be able to touch out at the end and could be charged a maximum fare. In addition, flat batteries could see penalty fares levied against those approached by inspectors.TfL is also telling Apple Pay users to always use the same device to avoid incomplete journeys. Meanwhile, contactless and Oyster cards should be kept out of phone cases to guard against card clash.Meanwhile, on Twitter there have been grumblings about the slowness of Apple Pay compared to Oyster, as well as the odd complaint about using Touch ID in the sweltering summer heat.