Updated 6.03pm: A disillusioned former member of Islamic State has passed a stolen memory stick of documents identifying 22,000 supporters in more than 50 countries to a British journalist.

The memory stick was handed to Sky News correspondent Stuart Ramsay.

"There has never been a leak from an active terror organisation on a scale even remotely like this."

He described it as "an unprecedented cache of information that could destroy Islamic State's fighting infrastructure. Tens of thousands of files, names, addresses, telephone numbers and crucially, the network that recruited them."

"To join up, would be recruits had to convince their sponsors of their eligibility. A form was crucial - 23 questions, with names and addresses, education, the countries they've travelled through on the way to Syria, previous Jihadi experience, and their phone number contact at home." 

It's a leak that could help the West target Islamist fighters planning attacks.

Leaks of such detailed information about IS are rare.

It could give Britain's spies a potential trove of data that could help unmask militants who have threatened more attacks like those that killed 130 people in Paris last November.

The leaker was a man calling himself Abu Hamed, who is a former member of Islamic State who became disillusioned with its leaders.

He said he had stolen the memory stick from the head of the group's internal security force.

Former British security and counter-terrorism minister Baroness Neville-Jones said there could be severe consequences inside ISIS.

"It's extraordinary that they should keep information of this kind on a memory stick, assuming that it really is the case, this is the sort of thing for which heads roll in organisations."

British security experts are calling the information a "goldmine".