The names of 22,000 ISIS supporters have been uncovered in documents handed to Sky News by a disillusioned former member of the militant group.
Tens of thousands of documents were stored on a memory stick stolen from the head of Islamic State's internal security force by a man who identified himself as Abu Hamed.
Originally a member of the Free Syrian Army, Hamed later joined ISIS but has now left the group, claiming "the Islamic rules he believed have totally collapsed inside the organisation," Sky reported.
The documents looked like enrolment forms with 23 questions and contained names of Islamic State supporters and of their relatives, telephone numbers, hometown and blood group and other details such as the subjects' areas of expertise and who had recommended them.
Sky News said some of the names were already well-known, but the documents could also help identify some extremists who were previously unknown to the authorities in their countries.
The names include more than a dozen Britons, including Abdel Bary from London, who joined ISIS in 2013, and Reyaad Khan from Cardiff, who also joined in 2013.
According to Sky, one of the files was marked 'Martyrs', and gave details of militants who were specifically trained to carry out suicide attacks.
German intelligence said it had obtained 22,000 documents identifying ISIS militants on Monday. They appear to be the same documents as obtained by Sky.
Germany's interior minister Thomas de Maizière said the documents would help to clarify "the underlying structures of this terrorist organisation".
Additional reporting by Reuters.