Documents containing the names, addresses, telephone numbers, and family contacts of 22,000 Islamic State (IS) extremist group supporters have been obtained by Sky News.
Sky News said it received the documents from a disgruntled, former member of IS, who reportedly stole them from the head of the group's internal security police.
It said they include the details of nationals from at least 51 countries.
Sky News said it had informed the authorities of the intelligence haul.
No comment was immediately available from Britain's interior or foreign ministries.
Copies of the documents broadcast by Sky News showed that recruits had to answer 23 questions including on their blood type, mother's maiden name, "level of Shari'a understanding," and previous experience.
The documents were stored on a memory stick by a man identified as Abu Hamed, a former fighter for the Free Syrian Army who defected to IS.
He then became disillusioned with the group, saying the Islamic rules that inspired him to join had collapsed inside the group.
Based on reporting by Sky News, dpa, and AFP