Europe faces tricky balancing act over its departing jihadists

"The systems to share information exist, but they are not always used," said Camino Mortera-Martinez of the Centre for European Reform think tank.

"There are very different parts of Europe and they don't necessarily trust each other. MI5 may not want to give information to Romania, for instance, because they are worried about corruption and information leaking out."A major sticking point has been sharing airline passenger information, which has been stalled since 2011 over concerns about data privacy.

Although 15 of 28 EU members have adopted their own data sharing systems, modelled on existing deals with the United States, Canada and Australia, the European Parliament insists that data protection laws must be adopted before the whole continent follows suit.

"If terrorists are a bit clever, they can find loopholes," said Mortera-Martinez.

"Given that agencies within the same country have trouble sharing information with each other, you can imagine how reluctant they are to share information with Europol or some other international body," she added.

"Intelligence services are not naturally very open people."