Plans to extend pay-as-you-go contactless ticketing to another 47 stations in south east England on 22 September have been delayed because of the cyber attack on computer systems at Transport for London.

TfL has also disclosed that financial and other data about its customers could have been obtained by the attackers, and that some services may be disrupted. It is reported that up to 5000 people could have been affected.

The National Crime Agency has revealed that a 17-year old boy was arrested in Walsall on 5 September in connection with the incident. He has since been bailed.

Chief technology officer Shashi Verma said: ‘The security of our systems and customer data is very important to us. We continually monitor who is accessing our systems to ensure only those authorised can gain access.  We identified some suspicious activity on Sunday 1 September and took action to limit access. A thorough investigation continues 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. This includes some customer names and contact details, including email addresses and home addresses where provided.

‘Some Oyster card refund data may also have been accessed. This could include bank account numbers and sort codes for a limited number of customers. As a precautionary measure, we will be contacting these customers directly as soon as possible to advise them of the support we can provide and the steps they can take.

‘We have notified the Information Commissioner's Office and are working with our partners to progress the investigation. We will provide further updates as soon as possible.

‘In addition, as part of the measures we have implemented to deal with the cyber incident, we have today put in place additional measures to improve our security. This includes an all-staff IT identity check. Throughout this planned process we have ensured that all safety critical systems and processes have been maintained.

‘We do not expect any significant impact to customer journeys as we carry out this process. However, temporary and limited disruption is possible to some services so, as ever, please check before you travel.’

‘The security measures we are taking mean that it is now not possible for us to deliver the necessary system changes to enable 47 additional stations outside London to benefit from pay as you go with contactless on 22 September as planned. We are working with DfT and the Rail Delivery Group to reschedule and we apologise for the delay.

‘We will continue to keep our customers and our staff updated. I would like to apologise for the inconvenience this incident may cause customers and I thank everyone for their patience as we respond to this incident.’