Facebook has said “almost 50 million” of its users were left exposed by a security flaw.

The company said attackers were able to exploit a vulnerability in a feature known as “View As” to gain control of people's accounts.

The breach was discovered on Tuesday, Facebook said, and it has informed police.

Users that had potentially been affected were prompted to re-log-in on Friday.

The flaw has been fixed, wrote the firm’s head of security, Guy Rosen, adding all affected accounts had been reset, as well as another 40 million "as a precautionary step".

Facebook - which saw its share price drop more than 3% on Friday - has more than two billion active monthly users.

Who has been affected?

The firm would not say where in the world the 50 million users are, but it has informed Irish data regulators, where Facebook's European subsidiary is based.

Users that had potentially been affected were prompted to re-log-in on Friday. However, the company said users did not have to change their passwords.

"Since we’ve only just started our investigation, we have yet to determine whether these accounts were misused or any information accessed. We also don’t know who’s behind these attacks or where they’re based. “

He added: "People’s privacy and security is incredibly important, and we’re sorry this happened." (BBC)