Large parts of Britain were hit by a power cut on Friday evening (local time), affecting trains, knocking out traffic lights and disrupting an airport.
London, the southeast and northwest of England and Scotland were impacted although there was no immediate word on the cause of the problem.
Train services in and out of London, including Thameslink, Southern and Gatwick Express were facing delays and cancellations. National Rail said "a large number" of train services had been affected.
Transport for London 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 and gas transmission system in England and Wales with day to day responsibility of balancing supply and demand.
No comment from National Grid was immediately available.
The electrical grid operator in north east England reported multiple power failures across the region, which affected the airport and metro system of the region's biggest city, Newcastle.
Western Power Distribution which serves the Midlands, South West and Wales said there had been a "major incident" but all affected customers now had power restored.
In Cheshire, northwest England, police tweeted that they were aware of a power outage in the Ellesmere Port area.