The UK’s National Trust is among more than 80 organisations that have confirmed data breaches resulting from an attack on cloud computing provider Blackbaud.
Others involved include homeless charities The Wallich and Crisis, the terminal illness charity Sue Ryder, and the mental health group Young Minds.
Dozens of British universities have also alerted past and present students.
Museums, schools, churches and food banks have also been affected.
The UK’s Information Commissioner’s Office (ICO) has said it is investigating the matter and is therefore limited in what it can say at this time.
The National Trust said that data about its volunteering and fundraising communities had been involved, but not that of its wider 5.6 million members.
The organisation – which looks after historic buildings and gardens – added that an internal investigation