Shares of Equifax Inc (EFX.N) tumbled 13 per cent on Friday after the provider of consumer credit scores revealed that personal details of as many as 143 million Americans were likely accessed in one of the largest data breaches in the United States.

The cyber attack comes nearly two years after rival Experian Plc, the world's biggest consumer credit monitoring firm, reported a data breach that exposed sensitive personal data of some 15 million people.

Atlanta-based Equifax said on Thursday it discovered the breach on July 29. It said criminals exploited a vulnerability in a website application to gain access to certain files that included names, Social Security numbers and drivers' license numbers.

"Obviously the size and scope of this breach will likely drive a number of negative headlines for EFX that will weigh on its brand for the foreseeable future," Barclays analyst Manav Patnaik wrote in a note.

Consumers' accounts were accessed by hackers between mid-May and July, Equifax said on Thursday, and added that information of some UK and Canadian residents was also obtained in the hack.

Britain's Information Commissioner's Office (ICO) said the breach "gives us cause fcoor ncern". ICO Deputy Commissioner James Dipple-Johnstone said the office would advise Equifax to alert affected UK customers at the earliest opportunity.

Equifax shares, which have gained 21 per cent this year, were set to open at their lowest in seven months on Friday. The stock was trading at $123.50 minutes before the bell.

"We expect Equifax and, to a lesser extent, Transunion and Experian, to face pressure near-term given the magnitude of the breach and attention on data integrity," analyst Michael Tarkan from Compass Point wrote in a note.

Equifax handles data on more than 820 million consumers and more than 91 million businesses worldwide and manages a database with employee information from more than 7,100 employers, according to its website.