An investigation was underway on Friday night after the personal data of up to 44 million British consumers was feared stolen by hackers in a massive cyber attack.

The information commissioner said it was investigating how the hack on Equifax, a US credit rating firm, affected UK customers, many of whom will be unaware their data is held by the company.

Equifax and its UK subsidiary companies state on their websites that they represent British clients including BT, Capital One and British Gas.

There are fears that customers of these companies could now be affected. BT said that "many companies in the UK" used Equifax services and said that it was "monitoring the situation closely".

The Information Commissioner's Office (ICO) has urged Equifax to alert affected UK customers as soon as possible, and said it will work with the relevant overseas authorities on behalf of British citizens.

Equifax says it holds the personal details of 44 million UK citizens but many British victims will be unaware they have had details stolen as they will not directly be Equifax customers.

Equifax admitted hackers had exposed the personal data of 143 million customers in the US, which was stolen between mid-May and July this year due to a vulnerability on its website.