Applications for 18+ Student, 18-25 Care Leaver, and Apprentice Oyster photocards have restarted.
Transport for London (TfL) announced the resumption of these applications as it recovers from a 'highly sophisticated' cyber security incident.
The breach, identified on September 1, led to the temporary suspension of the photocard application website.
TfL said it took this measure to safeguard network and customer data.
With assistance from the National Crime Agency and the National Cyber Security Centre, TfL worked to restore its services.
The Information Commissioner's Office was also informed of the breach.
The Deputy Mayor for Transport, Seb Dance, said: "TfL has been working incredibly hard around the clock to restore the services which had to be temporarily taken offline while it responded to a highly sophisticated cyber attack.
"The Mayor and I commend TfL on its response, which has kept London moving and avoided the consequences of the attack being even worse.
"We’re really pleased TfL has now started accepting Oyster photocard applications for several concessions, including 18+ Student and 18-25 Care Leavers, and are assured work is ongoing to ensure that other photocard applications are opened as quickly as possible in the coming weeks."
From November 4, TfL began processing applications for the 18+ Student, 18-25 Care Leaver, and Apprentice Oyster photocards.
The 18+ Student photocard offers a 30 per cent savings on adult-rate Travelcards and Bus & Tram Pass season tickets to eligible customers.
Shashi Verma, the Chief Technology Officer at TfL, said: "Following the recent highly sophisticated cyber security incident, we have been working to restore systems which we had to take offline as part of our mitigations.
"We’re now pleased that we can restart processing Oyster photocard applications for students and will also work to ensure that other photocard applications can start being processed as quickly as possible.
"Once customers have successfully applied for a new Oyster photocard, we will also contact them to advise on how to claim for any additional travel costs incurred while the photocard website has been unavailable."
The president of the Students' Union at University College London (UCL), Goksu Danaci, said: "The 18+ Student Oyster is a vital initiative for our students and we're relieved this issue, caused by a recent cyber-attack, is coming to an end.
"Students have had a difficult start to the academic year with continuing cost of living pressures, and travel costs are a large part of a student's daily expense.
"We're pleased TfL is now accepting 18+ Student Photocards applications again and has committed to refund students for any additional costs they've incurred during the time applications have been unavailable."
TfL has plans to begin accepting new applications for other concessions, including the 60+ Oyster and the 5-10, 11-15, and 16+ Zip Oyster photocards, later this month.
These will be phased, with expired 5-10 and 11-15 Zip Oyster photocards remaining valid until December 31, 2024.
TfL also intends to refund customers who incurred additional travel expenses due to the suspension of the photocard application service.
For the latest information on applications, visit the TfL website.
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