The attack, first reported by the Financial Times, comes after seven countries, including the U.S. and Britain, in June named Lockbit as the world's top ransomware threat.
"We have experienced a data incident impacting a small number of storage servers," an Allen & Overy spokesperson said in a statement.
"Investigations to date have confirmed that data in our core systems, including our email and document management system, has not been affected.
"The firm continues to operate normally with some disruption arising from steps taken to contain the incident."
(Reporting by Sam Tobin; editing by Sarah Young)
