Transport for London said it was dealing with a cyber attack.
The organisation, which runs much of London's transport network, said it had taken immediate action to prevent further access to its systems and was working closely with relevant government agencies, including the National Crime Agency and the National Cyber Security Centre.
TfL said it had found no evidence that customer data had been leaked and that transport services were not affected.
Shashi Verma, TfL's chief technology officer, said: “We have introduced a number of measures into our internal systems to address this ongoing cyber security incident. The security of our systems and customer data is extremely important to us and we will continue to assess the situation during and after the incident.”
“A full assessment needs to be completed but there is currently no evidence that customer data has been compromised. There is currently no impact on TfL services and we are working closely with the National Crime Agency and the National Cyber Security Centre to respond to the incident.”
After newsletter promotion
Sources told the BBC that it was mainly backroom systems at the transport agency's headquarters that were affected, and staff were being asked to work from home where possible.
