Holiday Inn owner IHG said on Tuesday 6 September that bookings on its websites and apps were facing disruptions after its technology systems were hit by "unauthorised activity".
IHG said it was assessing the nature, extent and impact of the incident and had implemented its response plans.
The company, which has appointed external specialists to investigate the incident and is in the process of notifying regulatory authorities, said its hotels were still able to operate despite the disruption.
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The incident comes amid increased scrutiny on appropriate defences against cyber attacks, particularly on western financial institutions, in the wake of heightened geopolitical tensions following Russia's invasion of Ukraine early this year.