Parts of London, the southeast of England and Wales have been hit by a large-scale power cut, meaning some traffic lights are not working, according to Transport for London (TfL). "Please be very cautious on the roads!" TfL said on Twitter. The problem is reportedly caused by a National Grid failure. "We're aware of a power cut affecting large parts of London and South East. We believe this is due to a failure on National Grid's network, which is affecting our customers," UK Power Networks said in a tweet. "We are currently experiencing a major incident on the national electricity infrastructure. We are in the process of restoring customers now and will provide further updates as soon as possible for specific areas. We are sorry for the inconvenience this is causing," Western Power Distribution (WPD), another electricity distribution operator said in a statement. London and large parts of England and Wales were hit by massive power cut on Friday after a suspected national grid failure, UK power companies said Friday. "We're aware of a power cut affecting a large area of London and South East. We believe this is due to a failure to National Grid's network, which is affecting our customers," UK Power Networks tweeted, later replying to customers confirming that "the issue was caused by a fault on the National Transmission Network," and that they expect power to be restored. Western Power Distribution, which distributes power to the Midlands, the South West region and Wales said that they were experiencing a "major incident on the national electricity infrastructure," but added that 90% of their customers have since had their power restored. "We are working hard to restore the remaining customers as soon as possible," WPD added in a tweet. Transport for London (TfL) has also confirmed to CNN that parts of its train network were affected following the power outage on Friday evening, but that power had been restored and service was expected to be back up and running shortly. According to Britain's Press Association (PA) news agency, a spokeswoman for TfL said police officers may need to operate busy intersections where traffic lights had lost power. She added that it was unclear how big the power cut was, but it was known that some traffic lights were "not working," PA said. Londoners shared pictures of the power surge on Twitter, which showed the tube network in darkness. The outage also affected several overground train services, with Thameslink tweeting that its trains had been brought to a "standstill," and London North Eastern Railway said several of its had broken down, according to PA. "Power supply problems are currently causing disruption to a large number of train services. Information screens in some areas are also affected," National Rail Enquiries said according to PA. Earlier on Friday, UK Power Networks warned that they were "preparing for forecast of strong winds" across the South East and East of England, directing customers to their website for further guidance on how power might be affected.