Dr Antony McRoy's speaking after the defector approached Sky News with a pen drive containing more than 22,000 files with addresses, phone numbers and family contacts of jihadis who have joined the group.
Nationals from more than 51 countries including the UK filled in a 23-question "registration" form as they were inducted into IS.
Dr McRoy told Premier's News Hour: "If it's genuine, this could be a very great help.
"What the security services will really be looking at would be: where they came from, who their family is, what were the communities, what mosque did they attend, what jobs did they have, what college did they go to, who were there friends or their contacts?
"After all, once these people go out to the Islamic State and stay there, then there's no big issue knowing who they are.
"The Russians and NATO are already bombing them... They're advancing against the Islamic State gradually.
"What particularly in the light of things like the Paris massacre the security services will be looking at is: who are the contacts of these people in the West?"
Listen to Premier's Hannah Tooley speaking to Dr Antony McRoy on the News Hour:
