Rail passengers heading to Heathrow Airport faced "major disruption" after a burst water main caused flooding in the tunnels.
The disruption began early on Saturday (May 30) and lasted until 1pm, affecting both Elizabeth line trains and Heathrow Express services.
This was due to a burst water main near the airport, which flooded the tunnels used by trains and caused issues to the signalling equipment in the area.
While lines have now reopened, trains running to and from Heathrow may still be cancelled or revised while service recovers.
Elizabeth line services to Terminal 4 are terminating at either West Drayton or Hayes and Harlington, while services to Terminal 5 are starting or ending at London Paddington.
Heathrow Express trains between London Paddington and Terminal 5 are currently suspended.
Passengers can use their tickets at no extra cost on either South Western or Great Western Railway on any reasonable route.
Those travelling from Reading can also take the RailAir RA1 coach service to Heathrow at no extra cost.
Travellers are being advised by National Rail to allow extra time for their journeys, and that replacement vehicles are expected to be busier than usual.
Network Rail engineers are on site working to resolve the issue, but due to the "severity" of the flooding and issues affecting the signalling equipment, this is expected to take the majority of the day today, National Rail said.
Those experiencing delays may be entitled to compensation and are advised to keep their tickets and note details of their journey.
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