The British Library has announced that an October cyber attack resulted in a leak of staff data.
The attack, which occurred on October 31, has also caused the library's website to remain down for over a month.
The Rhysida ransomware group claims responsibility for the attack and plans to auction off the stolen data.
The cyber gang says the fee for data, which includes passport scans, is 20 bitcoins (£596,459).
"Following confirmation last week that this was a ransomware attack, we're aware that some data has been leaked," said the British Library, the UK's largest library, in a post on X. This looks to be from our own human resources files."
However, it said that it has "no evidence that the data of our users has been compromised" and that it has not been established that the material being auctioned off belongs to British Library personnel.
A representative for the National Cyber Security Centre (NCSC) said the organization was working with the library to "fully understand the impact" of the incident.
According to the report, "Ransomware is the key cyber threat facing the UK, and all organizations should take immediate steps to limit risk by following our advice on how to put in place robust defenses to protect their networks."
On Monday, the Rhysida ransomware group claimed responsibility for the attack and posted a photograph on its dark web leak site depicting different papers, some of which look to be HMRC employment contracts and passports.
The BBC has not confirmed if the data is accurate.