A leaked document has reportedly revealed the identities of 22,000 extremists fighting for so-called Islamic State.
The documents show the addresses, telephone numbers and family contacts of those who have signed up to join IS, Sky News said.
Militants from 51 countries - including the UK - revealed their personal information when they filled in a 23-question form.
The "goldmine" of information is "incredibly important" to the ongoing battle to persecute and identity IS militants, experts say.
- A 'goldmine of information'
Former global terrorism operations director at MI6, Richard Barrett, described the leak as a "fantastic coup".
"It will be an absolute gold mine of information of enormous significance and interest to very many people, particularly the security and intelligence services," he said.
One of the forms is said to reveal the details of Junaid Hussain, who was killed in an RAF strike last year.
It reveals his fighter name "Abu Hasayn Al Britani", his mother's maiden name, his date of birth, the fact that he has a secondary level education and was previously an "electronics specialist".
- Document stolen by 'disillusioned ex-IS militant'
The documents were stolen by a former member of the Syrian Free Army who joined IS and then became disillusioned, saying that it has been taken over by soldiers from the Iraqi Baath party of Saddam Hussein, Sky News said.
He said he took them from the head of IS's internal security police.
Sky News said it had passed the cache of documents on to the security services.
- Information from cache is 'incredibly rare'
Shashank Joshi, a senior research fellow at the Royal United Services Institute security think-tank, described the leak as "incredibly important".
He told the Press Association: