Thomson and First Choice have not yet been able to fix back office problems which began on Wednesday morning.
The technical problems are impacting payments systems and booking confirmations, although bookings can still be made online, by phone and through retail shops.
Parent TUI had been hoping to have everything back up and running by Wednesday evening, but the systems are still not operating properly.
"We are working to rectify the problem and the usual systems will be up and running as soon as possible," said a spokeswoman.
TravelMole was alerted to the issues by a subscriber who complained that although Thomson had processed his booking for a flight, it was unable to provide a confirmation.
Bryan Dredge was told that the operator’s systems was able to accept bookings but could not send any confirmations.
"It’s a shocking state of affairs for major company like Thomsom to have a system unavailability for this long," he said.
The spokeswoman said delayed booking confirmations would be a priority once the problems were fixed.
by Bev Fearis
