BT and Sky internet users could be caught up in the huge Yahoo hack, even if they’ve never knowingly used Yahoo.

Both of the internet companies have used Yahoo’s email services to power their own, meaning that the loss of Yahoo data has also lost BT and Sky data. Those users might not know that they were ever using Yahoo’s services.

Because Yahoo owns a huge number of companies and provide a wide range of services, hundreds of millions of people might not know that they are part of the hack. Flickr users, for instance, might also have been unknowingly hit by the attack.

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The advice that Sky and BT have given out is the same as those given to Yahoo users. People have been urged to change their passwords and watch for suspicious activity.

"At Sky, we take the security of our customers' data and information extremely seriously," the company wrote on the help page of its website.

"You may have seen that overnight Yahoo! announced that a copy of certain user account information was stolen from its company's network in late 2014. Yahoo! is the provider of sky.com email accounts.

"If you are a sky.com email holder, in line with the advice provided by Yahoo!, we advise that you change your passwords online and follow good password management practices.”

Also in the UK, the Information Commissioner Elizabeth Denham has said that “serious questions” will be asked of Yahoo and how the data came to be stolen.

"The vast number of people affected by this cyber attack is staggering and demonstrates just how severe the consequences of a security hack can be," she said,” she said.

"The US authorities will be looking to track down the hackers, but it is our job to ask serious questions of Yahoo on behalf of British citizens and I am doing that today.

"We don't yet know all the details of how this hack happened, but there is a sobering and important message here for companies that acquire and handle personal data.