The majority of Heathrow Express trains have been taken out of service after a ‘crack’ was reportedly found on one of its carriages.
The service will now be operated by Heathrow Connect as Express trains are recalled for precautionary checks.
This means the stopping Heathrow Connect service has been suspended until March 3.
The Heathrow Express service will continue to operate every 15 minutes, and anyone holding pre-bought Heathrow Connect tickets will be eligible to use the service.
As there is no business class service on the Connect trains, all travellers who have booked a ticket are entitled to a full refund.
Heathrow Express has cut its single fare from London Paddington to Heathrow from £22 to £17 to help compensate travellers who would otherwise have taken the cheaper Connect service, which costs £10.20.
Train drivers union ASLEF told The BBC a crack had been found in the underframe of a carriage during a routine inspection.
A spokeswoman for Heathrow Express said the fault was "passable" but that engineers had "decided to test every single train" as a safety precaution.
Heathrow Connect said that "Passengers who wish to travel to one of the stations on the Heathrow Connect route should take the 140 bus from Heathrow to Hayes and Harlington, followed by a Great Western Railway service to Paddington".
Heathrow Express Director Fraser Brown added that: “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.”
A further update will be provided by the rail operator on March 1.