Gatwick, Charles de Gaulle, Changi, Johannesburg, Zurich, Melbourne and Washington DC’s Reagan airports are affected, with airlines including British Airways and Qatar Airways experiencing problems with online check-in.

A spokesperson told TTG:  “Amadeus confirms that during the morning, we experienced a network issue that caused disruption to some of our systems. As a result of the incident, customers experienced disruption to certain services.

"Amadeus technical teams took immediate action to identify the cause of the issue and restore services as quickly as possible. That action is ongoing with services gradually being restored.

"Amadeus regrets any inconvenience caused to customers.”

British Airways’ website has been affected, with its customer support telling one customer: "This issue is not related to our website, it has affected most airlines with Amadeus system based."

Qatar Airways has told customers: "This is a system outage for many airlines using the Amadeus IT network. The Amadeus technical teams are working to bring back the service as soon as possible. We regret any inconvenience this may cause to our passengers."

And on Twitter, Heathrow Airport’s customer support account has said that "a small number of airlines are experiencing problems across the world & we’re working closely with them to solve the issue".

Meanwhile, Key Travel has issued an statement, concerning the use of the Amadeus Altea check-in software: "We are currently impacted by a global problem with Amadeus GDS (Global Distribution System). We are impacted by the issues with the GDS system and are currently unable to make new reservations or changes to existing reservations... We will continue to monitor the situation as it develops and provide updates to our clients as necessary."