London, the south east and north west of England and Scotland were impacted, leaving hundreds of thousands of people temporarily without electricity.
National Grid, which owns the electricity transmission system in England and Wales, said there had been "issues" with two power generators and the problems have now been resolved.
Members of the public posted images on social media, which showed the tube network in the darkness and people having to use their mobile phones as torches.
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.
The electrical grid operator in northeast 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.
Some people on social media reacted to the outage with humor.
"Was that #powercut someone attempting to fix Britain by turning it off and on again?" Emma Clarke said on Twiter.
Another person referencing the possibility that Britain face disruption if it leaves the European Union without a trade deal at the end of October said: "Country having a practice for no deal Brexit #powercut "