Around 400,000 people in the UK may have had their information stolen following a cyber security breach at credit monitoring firm Equifax.
The US company said an investigation had revealed that a file containing UK consumer information "may potentially have been accessed".
The data includes names, dates of birth, email addresses and telephone numbers, but does not contain postal addresses, passwords or financial information.
Atlanta-based Equifax discovered the hack in July, but only informed consumers last week.
In an effort to provide reassurance, the firm said it was unlikely that people would be hit by "identity takeover".
It said it would contact them in writing to offer advice and a free identity protection service monitoring their personal information and data.
Equifax president Patricio Remon said: "We apologise for this failure to protect UK consumer data.
"Our immediate focus is to support those affected by this incident and to ensure we make all of the necessary improvements and investments to strengthen our security and processes going forward."
Equifax alerted the public to the cyber attack on September 7. The attack saw the data of 143 million people breached in America.
In a statement on Friday, Equifax said its UK systems had not been impacted by the attack, but the information of British consumers may have been accessed because of a process failure in 2016 which saw a "limited amount" of UK data stored on the American system between 2011 and 2016.
It added that the UK consumer data that may have been stolen does not include "any single Equifax business clients or institution".
It comes after Equifax was ordered by the Information Commissioner's Office (ICO) to alert British customers after the firm said "criminals'" had exploited a website application to access its files.
Lenders rely on the information collected by credit bureaus such as Equifax to help them decide whether to approve financing for homes, cars and credit cards.
Cyber attacks have become an increasing problem for big firms that hold a large amount of customer data.
HSBC and TalkTalk are among the most high profile British firms to be hit in recent years.