Large parts of London and the southeast of England were hit by a power cut on Friday evening, disrupting trains and knocking out traffic lights.
Train services in and out of London, including Thameslink, Southern and Gatwick Express were facing delays and cancellations.
Transport for London (TfL) said the Victoria line on the city’s underground system had been suspended and warned people to take extra care when using the roads because some traffic lights were not working.
“We believe this is due to a failure on National Grid’s network, which is affecting our customers,” UK Power Networks said on Twitter.
National Grid owns the electricity & gas transmission system in England & Wales with day to day responsibility of balancing supply and demand.
No comment from National Grid was immediately available.