Following investigation, a fault – a structural defect on the underside of the carriage – has been found on some Heathrow Express trains.
The trains have been taken out of service for the foreseeable future causing disruption between London and the airport.
The trains will now undergo further examination and maintenance work.
Passengers can still reach the airport using the Heathrow Express as a 15-minute service is being maintained using alternative trains.
Pre-bought Heathrow Connect tickets will be accepted on the Heathrow Express, and the 140 bus between Heathrow and Hayes & Harlington.
They will also be accepted on the London Underground between Paddington and Heathrow.
Heathrow Express tickets, which will be reduced during this period, will be required to travel on the non-stopping service as usual.
In a statement the operator said: “Heathrow Express would like to apologise for the inconvenience this has caused customers, and thank both Siemens and CAF, who have sent engineers from Germany and Spain to assist with the investigations.”
A single ticket will cost £17, and a return will cost £28.
Heathrow Express Director Fraser Brown said: “The safety of our customers and colleagues remains our top priority.
“We apologise for the inconvenience this has caused so far, and assure passengers that we are doing everything we can to return to running a full and safe service, as quickly as possible.
“We would like to thank all our customers for their patience.”