The mayor wants rail services to accept Oyster cards by the end of 2009
London's mayor has been urged to ensure that rail passengers are not penalised when pay-as-you-go Oyster cards are allowed across the city's rail network.
London's mayor Boris Johnson wants most suburban rail services in London to use the cards by the end of 2009.
But he could not guarantee a passenger using an Oyster card on the overground network will not pay more than if they had made the same journey by Tube.
Critics described the admission as a "bitter blow" to rail travellers.
The mayor said that although he "shared the aspiration" for a single unified pay-as-you-go scale across London, fares on national rail services in London are set by the train operators.
Lib Dem members of the London Assembly (LA) said the mayor must do more.
"It is long overdue for pay-as-you-go to be accepted on all railway lines in our capital," said Caroline Pidgeon, the Lib Dem's transport spokesman.
"However, to discover that pay-as-you-go fares will not be the same as Tube travellers enjoy will come as a bitter blow to rail travellers in London."
She added: "Boris Johnson needs to ensure that Londoners will not be penalised by higher fares just because people take the train rather than the tube."
For example, between Finsbury Park and King's Cross it costs £2.20 on the Tube whereas it costs £4.00 on rail.