Train passengers in south London are unable to collect pre-paid tickets from vending machines and offices.
Rail users are advised to travel using their booking confirmation as proof of purchase.
In a statement, posted on Southern Rail's website, it said: "We have been made aware of a Govia Thameslink Railway wide issue preventing customers from collecting prepaid tickets from ticket vending machines and ticket offices.
"Customers are advised to travel with their booking confirmation as proof of purchase.
"We are working to get this resolved and would like to apologise for any inconvenience this has caused."
Southeastern advised people to try and collect their tickets as early as possible.
They tweeted: "There's a known issue with collecting pre-paid tickets from our ticket machines.
Please purchase/collect tickets at 1st available opportunity."