Both companies have been forced to contact subscribers asking them to change their passwords following the attack. Before it began migrating users to a new system in 2013, BT's email service was powered by Yahoo, which was known as BT Yahoo Mail at the time. A small number of people are still using those old accounts and could therefore be implicated in the breach.

In a statement, BT said: "A minority of BT Broadband customers have a legacy email product from Yahoo. We advise customers generally to reset their password regularly and we will be contacting affected customers specifically to help them keep their information safe."

It's a similar story with Sky, but the pay-TV provider still actively relies on Yahoo to provide its Sky Yahoo Mail service. Unfortunately, that means Sky customers are more likely to be affected by Yahoo's security woes. "If you use Sky Yahoo Mail we'd advise that you change your password to help keep your email account safe," the company says on its website.

With Yahoo admitting that security questions and answers were also grabbed in the hack, users should be extra vigilant if they are contacted by someone purporting to represent Yahoo, BT or Sky. Personal information should never be shared via email and always ensure that if someone makes a telephone enquiry, to get a confirmed telephone number to call them back on.