InterContinental Hotels Group PLC booking platforms and some other technology platforms continued to experience problems after a cyberattack on Monday.
The FTSE 100-listed owner of the Holiday Inn and Crown Plaza chains said on Tuesday afternoon that its systems "have been subject to unauthorised activity".
It said booking channels and other applications "have been significantly disrupted" since Monday.
A message on its website on Wednesday told customers they "may have challenges" booking new reservations and accessing information about upcoming reservations.
IHG said in its investor statement that it was working with its technology suppliers and hacking specialists as it looked to resolve the problem, as well as notifying regulatory authorities.
"IHG is working to fully restore all systems as soon as possible and to assess the nature, extent and impact of the incident. We will be supporting hotel owners and operators as part of our response to the ongoing service disruption," it said.