Ticketmaster has said some customers' personal information and credit card details have been stolen in a data breach.
The ticket sales website said in a statement that it was hit by "malicious software" on Saturday through a third-party supplier to Ticketmaster.
All customers the company believes were affected – less than 5% of its customer base – have been contacted and advised to change their password.
In a statement on their website, Ticketmaster added customers in North America were not affected.
The company offered 12 months free identity monitoring service to anyone whose details may have been stolen.
The statement said: "On Saturday, June 23, 2018, Ticketmaster UK identified malicious software on a customer support product hosted by Inbenta Technologies, an external third-party supplier to Ticketmaster.
"As soon as we discovered the malicious software, we disabled the Inbenta product across all Ticketmaster websites."
It went on to say: "As a result of Inbenta's product running on Ticketmaster International websites, some of our customers' personal or payment information may have been accessed by an unknown third-party."
A spokesperson for the National Cyber Security Centre said they were aware of the incident and working with partners to understand what happened.
Ticketmaster added that it is working with relevant authorities, as well as credit card companies and banks, and had its own forensics teams and experts analysing the breach.
Ticketmaster said the breach mainly affected UK customers, but that Ticketmaster International customers are also being advised to change their passwords.
Anyone who attempted to buy tickets between February and 23 June of this year, and international customers who bought or tried to buy tickets between September 2017 and 23 June 2018 could have been affected.
The software was running on Ticketmaster International, Ticketmaster UK, GETMEIN! and TicketWeb.
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Affected data includes name, address, email address, telephone number, payment details and Ticketmaster login details.
It is not clear how many people in the UK were affected, but according to LiveNation, which owns Ticketmaster, they sold more than 387,000 tickets per day globally in 2011.