Elizabeth Denham, UK Information Commissioner said:
"Yahoo! has indicated that significantly more people may have been affected by the data breach reported last December than first thought and that all user accounts may have been affected by the August 2013 theft. This is understood to include all UK Yahoo! account holders at the time. This gives us further cause for concern.
"It is very disappointing to see the company is apparently still uncovering additional problems despite the length of time since the breach occurred.
"We are talking to Yahoo! and have advised them to contact all customers affected as soon as possible.
"We continue to investigate alongside the relevant international authorities to ensure the data protection interests of UK customers are considered."