Google is hosting tutorials on how to hijack Facebook accounts using a similar method to the hackers who had gained access to the personal data of 50 million users.

The step by step video guides could still be accessed on YouTube, Google's popular video streaming website, hours after Facebook revealed the breach.

Experts warned that any number of other hackers, including foreign intelligence agencies, could have accessed people's accounts continually since July 2017.

Guy Rosen, Facebook's vice president of product management, told reporters that 50 million users – including Facebook's chief executive Mark Zuckerberg and its chief operating officer Sheryl Sandberg – were confirmed victims of the attack, but that an additional 40 million users might have been exposed to similar attacks.

On YouTube, the tutorials - some of which have been deleted by Google - explain how to hack into Facebook profiles by stealing "access tokens", digital keys which allow users to log in without entering their passwords every time. They have already been watched several thousands of times.

An attacker who has a user's access token can use their account as if they are that user, from posting in their name to reading through their messages to looking through an archive of what they have "liked" and shared.