The personal data of more than 500 million Facebook users has been shared on an online hacking forum.
The huge data leak involves the phone numbers and personal data of 533 million accounts from 106 countries.
Around 11 million users have been affected in the UK, compared with 32 million in the US and six million in India.
The details that have been leaked include Facebook IDs, birthdates, names, locations and in some cases email addresses.
Facebook told Business Insider that data was stolen as a result of a security issue that the company patched in 2019.
The 'data set' was shared on a hacking forum on Saturday.
Although the information is two years old it could still be used by scammers, according to cybercrime experts.
Alon Gal, Chief Technology Officer of internet security firm Hudson Rock, told Insider: "A database of that size containing the private information such as phone numbers of a lot of Facebook's users would certainly lead to bad actors taking advantage of the data to perform social engineering attacks (or) hacking attempts.
"Individuals signing up to a reputable company like Facebook are trusting them with their data and Facebook (is) supposed to treat the data with utmost respect.
"Users having their personal information leaked is a huge breach of trust and should be handled accordingly."
Mr Gal discovered the leak in January after spotting an advert from an automated bot which could supply phone numbers for hundreds of millions of Facebook accounts.
Now all of the data has been shared on a hacking forum for free.
A spokesman for Facebook told Bloomberg: "“This is old data that was previously reported on in 2019.
“We found and fixed this issue in August 2019.”