Transport for London (TfL) has said that some customers' bank account details could have been accessed by hackers during an "ongoing cyber security incident".
The transport operator first reported a cyber attack on 2 September.
TfL’s chief technology officer Shashi Verma said on Thursday that a "limited number of customers" may have had bank account numbers and sort codes accessed.
He said those impacted would be contacted directly.
Mr Verma said an investigation into the cyber attack was taking place alongside the National Crime Agency and the National Cyber Security Centre.
"Although there has been very little impact on our customers so far, the situation continues to evolve and our investigations have identified that certain customer data has been accessed," he said.