Some 175 jihadis may soon attempt to flee back to Spain to join an estimated 37 who have already made the trip, according to the Spanish Department of National Security.
The organisation warned Islamist terrorism is “one of the main global threats” and said Madrid must step up action to stamp out jihadist propaganda online to reduce the number of people who are radicalised at home.
The Department of National Security said as groups like ISIS and al-Qaeda could begin to focus on carrying out attacks in Europe as its fighters are driven out of countries like Syria and Iraq, Madrid-based newspaper ABC reports.
A report by the authority said there are an estimated 50,000 jihadists who travelled to fight in the Middle East – some 6,000 of which were from Europe.
The department estimated around a third of foreign fighters have already returned to their country of origin.
In addition to veteran fighters returning from combat in the Middle East, terror groups are also continuing in their efforts to recruit young people through the internet, the study said.
The findings of the Spanish Department of National Security mirror the situation in the UK, where hundreds of British jihadis are believed to have already returned home after making the trip to Syria and Iraq.
Shocking research by The Soufan Centre in October last year revealed at least 425 ISIS members had already fled the ruins of the self-declared caliphate.
Some had reportedly become disillusioned with the terror group, while others were simply retreating from the region after US-backed forces drove them out of Islamic State’s Syrian capital of Raqqa.
Following the revelations, Defence Secretary Gavin Williamson said Britons who had fought for the Islamist group should be hunted down and killed to ensure they do not return to the UK.
Shortly after taking up his senior cabinet role, he told the Daily Mail: “A dead terrorist can’t cause any harm to Britain.”
“I do not believe that any terrorist, whether they come from this country or any other, should ever be allowed back into this country.
“We should do everything we can do to destroy and eliminate that threat.”
His incendiary comments put him directly at odds with Max Hill, the head of the terrorism watchdog, who said the UK could attempt to reintegrate young and naive jihadists who wanted to return to Britain.
Mr Hill QC, the independent reviewer of terrorism legislation, warned of the danger of “losing a generation” of men and women by automatically using the courts to punish them as soon as they returned to the UK.
Additional reporting by Maria Ortega.