Trains have been disrupted between London Paddington, Heathrow Airport and Reading on Sunday morning after damage to overhead cables.
The electric outage means that Heathrow Express, Elizabeth Line and Great Western Railway (GWR) trains will all be affected.
GWR said at around 9am that no trains were operating between Paddington and Reading, although alternative routes were in place for some passengers.
National Rail has said the disruption will last until midday on Sunday.
A statement read: “Damage to the overhead electric wires between London Paddington and Heathrow Airport / Reading is causing disruption to journeys running to and from these stations.
“As a result, trains may be cancelled, revised or delayed by up to 60 minutes.”
For the Elizabeth Line, there is no service between London Paddington and Ealing Broadway. Stations between Ealing Broadway and Heathrow/Reading are receiving a reduced service.
National Rail advised customers to check before they travel.
A statement added: “You may be entitled to compensation if you experience a delay in completing your journey today. Please keep your train ticket and make a note of your journey, as both will be required to support any claim.”
Tube strikes begin in London
The delays come hours before industrial action is set to begin on London Underground lines, with the Piccadilly Line set to be down when Arsenal play Liverpool at home on Sunday.
Businesses are fearing they could lose up to £50m as a result of a series of Tube strikes which will run until January 12. RMT, the union behind the strikes, has said that they are striking in response to feared job cuts across the network.
