The company said sensitive information of these British consumers was involved, according to a Reuters report.

This was not part of the 145.5 million users who were disclosed as having their credit data put at risk of exposure in the hack which the company announced on 7 September had taken place between mid-May and July.

It said the 693,665 British consumers would be notified of the post and offered help to mitigate the effects of the leak.

“Once again, I would like to extend my most sincere apologies to anyone who has been concerned about or impacted by this criminal act,” said Patricio Remon, Equifax’s Europe president.

“Let me take this opportunity to emphasise that protecting the data of our consumers and clients is always our top priority."

Security firm Mandiant, which has been hired by Equifax to investigate the breach, has said the hack took place in March.

The Mandiant report was sent to some customers, among them financial firms. It said that, as already reported, the attackers had taken advantage of a vulnerability in the Apache Struts Web application framework to gain entry.

The flaw was revealed two days before the first infiltration of Equifax. The details were released by Cisco's Talos intelligence unit.