A power cut hit parts of London and Southeast England, shutting down many traffic lights across the country, TfL reported on social media.
“Please be very cautious on the roads!” TfL said on Twitter.
Police officers could be forced to man busy junctions where traffic lights have been shut down by a power cut, a spokeswoman confirmed.
“We’re just assessing how many traffic signals are out,” she said, adding that some of TfL’s systems which control traffic lights were also “not operating”.
Large parts of the country are without electricity following the power cut, UK Power Networks said.
“We’re aware of a power cut affecting large parts of London and South East,” the company tweeted.
“We believe this is due to a failure on National Grid’s network, which is affecting our customers.”
Western Power Distribution shared a similar message, and said they are in the process of restoring power to customers.
Train services in and out of London, including Thameslink, Southern and Gatwick Express are facing delays and cancellations.