Rail connections between London and Heathrow face disruption until midday after passengers were left stuck on trains for hours without power on Thursday night.
Damage to overhead cables blocked routes to and from Paddington station.
The Elizabeth Line, Great Western Railway and Heathrow Express were all affected.
National Rail warned that “major disruption” was expected until 12pm today.
Passengers were urged to check services before travelling or seek alternative methods of reaching the airport.
Transport for London warned early this morning that there was no Elizabeth Line service to Heathrow while Network Rail repaired damaged overhead power cables in the Paddington area.
Heathrow Express trains were halted until at least 5.30am this morning. Services in both directions face delays of up to 15 minutes or will be subject to cancellations throughout today due to urgent repairs to the track.
Heathrow said: “Due to an ongoing incident, Heathrow Express services to and from Heathrow on Friday 8 December may be impacted.
“We advise passengers to allow extra time for their journey and apologise for any inconvenience caused.”
Transport for London told BBC News that four Elizabeth Line trains were left stranded after “another rail operator’s train has caused significant disruption to our Elizabeth line customers”.
They added: “Network Rail are working urgently to repair the power lines and we’d encourage all customers to check before they travel over the next few days while they do this.”
A spokesperson for Network Rail apologised for the “disruption and difficulties passengers endured” on Thursday evening after a fault with overhead cables powering trains in the Ladbroke Grove area.
National Rail said: “Yesterday a train struck an obstruction between London Paddington and Acton main line, causing damage to the overhead electric wires.
“Trains are not able to run through the affected area as the overhead power is switched off.
“Specialist engineers have been arranged to attend the location of the fault and we are waiting for an update on how long it will take to fix the problem. Due to the high risk nature of the repair work it may take a considerable amount of time for repairs to be completed.”
Early morning services by the Heathrow Express on Sunday have also been cancelled until 7.15 am due to separate planned railway upgrading work.