Heathrow Express has been forced to take most of its trains out of service as a safety precaution after routine maintenance identified a technical issue.
The train operator said that the majority of its trains had been recalled and would be thoroughly inspected by a specialist team.
However, it said it would continue the non-stop service leaving every 15 minutes between Paddington and Heathrow for the next 48 hours, served by class 360 units usually seen on the Heathrow Connect service.
The number of tickets available on the Heathrow Express will be reduced and Heathrow Connect will be suspended during this period.
Passengers who wish to travel to one of the stations on the Heathrow Connect route should take the 140 bus from Heathrow to Hayes & Harlington, followed by a Great Western Railway service to Paddington.
Heathrow Express director Fraser Brown said: "While we regret the inconvenience this will cause to our customers and airport colleagues, safety remains our top priority.
"We are working with Siemens engineers, who manage our depot site and the maintenance of our trains, to understand the extent of this issue. When we are aware of what we are dealing with, we will be able to make a decision on when a full service will resume."
The operator said it would give a further update tomorrow.
