London - The alleged identities of some 22,000 Islamic State militants have been revealed in a cache of documents, according to media reports on Thursday, although analysts cast doubt on their authenticity.

The information, which some experts said would deal a blow to the IS group, was reportedly included in forms featuring 23 questions which new recruits had to fill out in order to be accepted into the group.

The documents contain details like names, dates of birth and phone numbers for people from 51 countries including from Britain, northern Europe, the Middle East, north Africa, the United States and Canada.

If the documents are proved to be genuine, experts said they could help intelligence services around the world track down people who have travelled to countries such as Syria and Iraq to join IS.

“What’s important now is that the authorities can look at how this information can be used in the fight against Daesh and if it can then we would welcome that,” British Prime Minister David Cameron’s spokeswoman told reporters at a daily briefing. She said the government was not aware of the story before the reports came out.

The size of the cache was revealed by Britain’s Sky News television following German media reports earlier this week about a questionnaire which new IS recruits from Germany had to fill in.

The documents on German suspects are believed by authorities there to be authentic and German Interior Minister Thomas de Maiziere said they would help “a better understanding of the structures” of IS. He said it would also pave the way for “faster, clearer investigations and higher prison sentences”.

Richard Barrett, formerly a senior figure in Britain’s MI6 overseas intelligence service, said the leak would be “an absolute gold mine of information of enormous significance and interest”.

But leading experts pointed out mistakes and uncharacteristic language in the forms. “There would be big alarm bells for me, because when I’ve seen inconsistencies like that in the past they’ve been on really shoddily-made forgeries,” Charlie Winter, a researcher at Georgia State University, told AFP.

Moreover, in France, more teenage girls than boys are drawn to joining Islamic State jihadists in Iraq and Syria, a high-ranking anti-terrorist official told AFP this week.

“Among minors, females are over-represented to a proportion of 55 percent” of those interested in making the journey, or who have already done so, the French source said on condition of anonymity. Like young girls across Europe who dream of reaching Syria — and often leave their unsuspecting families shocked when they do run away — these girls are not just dreaming of becoming meek so-called “jihadi brides”.

While marriage to a jihadist fighter is their likely fate, the girls are as attracted by violence as their male counterparts, said sociologist Farhad Khosrokhavar, who has interviewed many radicalised French teenagers of both sexes. “Previously, violence was almost exclusively a male phenomenon (but) this generation has a different outlook,” Khosrokhavar said.

The Arabic name for “The Islamic State in Iraq and Syria,” IS’s previous nomenclature, is written in two different ways, including one that is not consistent with past practice. Files documenting the deaths of IS militants use the words “date of killing” instead of the typical militant term “martyrdom.”

Romain Caillet, an independent jihadism expert, also noted that some documents feature a second, circular logo not previously used on IS files.

Syrian opposition news website Zaman al-Wasl said there were thousands of repetitions in the leaked documents and the names of only 1,700 people could be identified in the 22,000 documents.

Contacted by AFP, a spokesman for Britain’s Home Office said the ministry would not not comment on national security issues.

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.

Questions on the documents include asking recruits their blood type, mother’s maiden name, “level of sharia understanding” and previous experience, it said.

Names on the list include Reyaad Khan, a 21-year-old British militant who Britain’s government said was planning attacks on Britain before being killed by a British drone in Syria last year.

Another on the list was Junaid Hussain, a British computer hacker described by British authorities as a key IS operative, who died in a US air strike last year.

The alleged leak comes with Western security services on high alert against the possibility of fresh militant massacre following the Paris attacks spree last November.

On Monday, Britain’s most senior counter-terrorism police officer warned of the risk of “spectacular” attacks targeting the “Western lifestyle”.

“You see a terrorist group which has big ambitions for enormous and spectacular attacks, not just the types that we’ve seen foiled to date,” said Mark Rowley, assistant commissioner at London’s Metropolitan Police. British police have previously said they foiled seven plots between late 2014 and early 2016.