Heathrow Express has been forced to recall trains for inspection after a routinely maintenance check found "cracks" in an undercarriage of a train.
A Heathrow Express spokeswoman told getwestlondon the engineers had decided to test every single train as a safety precaution even though the fault was "passable".
Trains from the Heathrow Connect services have instead been dispatched to assist with the shortage.
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."
Tickets available for the Express have been reduced as a result and passengers are expected to experience delays.
Commuters took to Twitter to show their frustration at the delays, claiming the news was "the perfect storm".
Alternative routes
Heathrow Express has cut its single fare from London Paddington to Heathrow from £22 to £17 to help commuters with the cost of the more expensive fair, which would otherwise be cheaper with the Connect services.
Great Western Railway will run extra trains between London Paddington and Hayes & Harlington.
Those wishing to travel along the Connect route have been advised to take the 140 bus from Heathrow to Hayes & Harlington, followed by a Great Western Railway service to Paddington.
