Around 800 extremists recruited by Islamic State or groups affiliated to al-Qaeda are planning to launch attacks in Europe, counter-terrorism sources in Spain have claimed.
The jihadists are said to have returned to the continent from conflict zones in Syria and Iraq and are waiting for instructions from support networks.
The counter-terrorism sources told Cadena Ser, a Spanish radio station, that some of the "returners" were being tracked but admitted they did not know the whereabouts of all of them.
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"These people are not well trained, but they are poised and prepared to do anything. Once they have been indoctrinated, they receive the minimum training," one unnamed source told the station.
The warning comes after Moroccan gunman Ayoub El-Khazzani was overpowered by members of the public on a train travelling from Amsterdam to Paris last week. The 25-year-old, who was arrested with a rifle, knife, pistol and ammunition, is believed to be an "Islamist returner" who left France for Syria last year.
Khazzani claims he was trying to commit a robbery and is not a terrorist, but authorities believe he had been armed and given instructions to carry out a massacre.
According to the Metropolitan Police, around half of the 700 Britons thought to have travelled to Syria and Iraq to support jihadist organisations have since returned. Some are being monitored, while others have been referred for action under Prevent, the counter-extremism strategy, reports The Times.
Earlier this year, Assistant Commissioner Mark Rowley, head of counter-terrorism at Scotland Yard, said "significant numbers" of extremists influenced by Islamic State were staying in the UK to plot attacks. In 2014, 165 arrests were made for Syria-linked terrorism offences across the UK, compared with 25 in 2013.
