Large parts of England and Wales were left without electricity following a major power cut that had a serious impact on rail and road services, including city traffic lights.

Passengers were shut out of some of the country’s busiest train stations during the Friday evening rush hour, while hundreds of thousands of homes were left without electricity after what the National Grid described as a problem with two generators.

British Transport police said officers were asked to help as services on the east coast mainline, as well as those into and out of St Pancras station were suspended, with many customers being advised not to travel. London’s Euston station, the southern hub for the west coast mainline, was also closed because of “exceptionally high passenger numbers”. The outage was also affecting other rail services and, reportedly, some traffic lights.

A National Grid spokesperson said the generator issues had caused “loss of power in selected UK areas”. They described the event as “unexpected and unusual”, saying: “Even though these events are outside of our control, we have plans in place to respond and the system operated as planned by disconnecting an isolated portion of electricity demand.

“We appreciate the inconvenience caused but this action allows the system to protect itself and limit the fall in frequency, allowing for power to be quickly restored. By 6.30pm [on Friday], all demand was restored by the distribution network operators. The system is operating normally.”

On Friday night, the energy watchdog, Ofgem, warned it could take “enforcement action” over the outage, saying it had demanded an “urgent detailed report from National Grid so we can understand what went wrong”.

About 500,000 customers in Wales, south-west England and the Midlands were affected and 300,000 customers in south-east England were left without power, the local distributors said. A further 110,000 in Yorkshire and north-east England were affected, alongside about 26,000 in north-west England, according to the electricity distributors in those areas.

Benny Wise (@BennyWise19) Kings Cross has just been evacuated.... pic.twitter.com/IeMwVHFr78

EnAppSys, an energy consultancy, said the blackout may have been caused by the unexpected shutdowns of the Hornsea offshore wind farm, which is owned by the Danish wind farm company Orsted, and the Little Barford gas-fired power plant, owned by German utility giant RWE.

National Grid data showed both of the generators dropped from the grid at around the same time. The twin outages caused a sudden loss of frequency of the electricity grid, to below 49Hz, which would have caused certain parts of the network to disconnect automatically, causing the power cuts.

“We would have expected the system to cope with this size of loss of generation,” an EnAppSys spokesman said. “This implies that there may have been [other] issues at the time of the trips.”

After the outage, a Transport for London (TfL) spokeswoman said some traffic lights were not working in the capital but the scale of the problem was not immediately clear. Police officers could be called in to “manage the busy junctions, to physically manage them themselves”, she said.

Traffic lights were also affected in Bradford, the bus operator First West Yorks said. One social media user said it was “like GTA [Grand Theft Auto] out here” as a result.

魔理沙 (@MarisaOrRisa) Flash flood took traffic lights out all across Bradford City Center.. Chaos on the roads near Godwin and Thornton. Like GTA out here. Every car and pedestrian for themselves! pic.twitter.com/9G25YdroJF

Thameslink, Southern and Gatwick Express were among those affected and National Rail said “a large number” of train services had been affected.

Thameslink (@TLRailUK) ⚠️ #TLUpdates - The power network has failed in the large parts of London and the South East. This has prevented our trains between Farringdon and Bedford from being able to take power and as a result, most of our trains are currently at a stand.



ℹ️👇https://t.co/5B8CgV5cH3

Members of the public posted images on social media showing the impact of the power outage, with one passenger publishing a video showing people climbing a set of steps in darkness at Clapham Junction.

Steph (@stefidoo) Welcome to Clapham Junction #powercut pic.twitter.com/sPppOOqRge

There were also power outages reported at Newcastle airport and in Cheshire police tweeted they were aware of an outage in the Ellesmere Port area, including in Little Sutton and Great Sutton.