The fine was deemed to be "reasonable and proportionate" under the Data Protection Act, according to the circumstances detailed in a 32-page statement accompanying the decision.
While the details of 145.5 million Americans were leaked, the company announced in October that, additionally, 15.2 million British client records were exfiltrated from its systems, affecting about 700,000 consumers.
Equifax said customer passwords were encrypted at time, but ICO notes customer passwords were stored in plain text files on a file server for UK fraud team. ?— Kevin Beaumont ? (@GossiTheDog) September 20, 2018
The ICO said its investigation had found that the US Department of Homeland Security had warned Equifax about a critical vulnerability as far back as March 2017.
"The ICO investigation found that, although the information systems in the US were compromised, Equifax was responsible for the personal information of its UK customers.
"The UK arm of the company failed to take appropriate steps to ensure its American parent Equifax, which was processing the data on its behalf, was protecting the information."
Information Commissioner Elizabeth Denham said: “The loss of personal information, particularly where there is the potential for financial fraud, is not only upsetting to customers, it undermines consumer trust in digital commerce.
“This is compounded when the company is a global firm whose business relies on personal data.
“We are determined to look after UK citizens’ information wherever it is held. Equifax Ltd has received the highest fine possible under the 1998 legislation because of the number of victims, the type of data at risk and because it has no excuse for failing to adhere to its own policies and controls as well as the law.”
She added: “Many of the people affected would not have been aware the company held their data; learning about the cyber attack would have been unexpected and is likely to have caused particular distress.
“Multinational data companies like Equifax must understand what personal data they hold and take robust steps to protect it. Their boards need to ensure that internal controls and systems work effectively to meet legal requirements and customers’ expectations. Equifax showed a serious disregard for their customers and the personal information entrusted to them, and that led to today’s fine.”
The American security firm Mandiant, which was hired by Equifax to investigate the breach, 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.