The mayor will make an announcement in the next few weeks
London commuters will be able to use pay-as-you-go Oyster cards on the city's suburban overland rail network by early next year, officials said.
Currently, the smartcard system can be used on the London Overground, the Tube, buses, trams and the Docklands Light Railway service.
The roll-out has been hit by delays, sparking criticism from campaigners.
Peter Hendy, London's transport commissioner, said there would be an announcement by the mayor "in weeks".
Kulveer Ranger, the mayor's transport adviser, said final details were being checked and said the scheme would be ready in early 2010.
Previously, Boris Johnson said most suburban rail firms would be able to use cards by the end of this year.
But this deadline was moved back to ensure that "Oyster was accepted" by the various train operators.
London TravelWatch described the delay as "unacceptable" and said the lack of an integrated system was "ridiculous".
There is also concern about how much passengers will be charged for using their Oyster cards on overland services.
In June, Mr Johnson said he could not guarantee a passenger using an Oyster card on the overland network would not pay more than if they had made the same journey by Tube.