Amadeus admitted some travel agents were still experiencing problems booking flights last night after its computer system crashed for three hours on Monday.
The global distribution giant apologised for the inconvenience but refused to say what caused the outage, which also affected passengers trying to check in online, reserve their seats or buy tickets.
Passengers trying to check-in for flights at airports were also delayed as staff were unable to scan passports so had to manually key in their details.
One report said the delays had almost brought flights in Finland to a standstill. Up to 150 airlines were affected, including British Airways, Iberia, Qantas, Cathay Pacific and SAS. Some flight delays were anticipated this morning as a knock-on effect.
Amadeus issued a two-line statement yesterday which said: "Amadeus can confirm that some airlines are experiencing difficulties with our systems. We regret any delay and inconvenience caused and are fully investigating this issue."
Last night it issued a further statement saying: "Amadeus can now confirm that all airline systems are restored.
"Some travel agency customers may continue to experience service disruption which we are investigating. We regret any inconvenience this may have caused."
It refused to say what caused the computer crash or how many passengers or travel agents had been affected.
By Linsey McNeill
