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Sky reported that a disillusioned former member had handed over the documents on a memory stick that had been stolen from the head of the group's internal security police.
The documents are forms that IS recruits had to fill out in order to be accepted into the organisation, and contain information on nationals from 51 countries, the broadcaster reported.
"Sky News has informed the authorities about the haul," the news channel wrote on its website. No comment was immediately available from Britain's interior or foreign ministries.(READ MORE)