Spanish counter-terrorism officials warned this week that around 800 Islamist militants are preparing to launch attacks around Europe.
The intelligence sources said that the extremists had mostly returned from Syria or Iraq where they had been recruited by Daesh or groups affiliated to al-Qaeda.
UK security agencies acknowledge that there are around 350 Britons who have returned from fighting in the Middle East and that some of them are plotting attacks here.
The Spanish warnings came after a Moroccan gunman was overpowered on a French train travelling from Amsterdam to Paris last Friday.
Three young Americans and a British grandfather prevented the attacker, Ayoub El Khazzani, 25, from carrying out a massacre. On Monday, President Francois Hollande presented them with the Legion d'honneur, France's highest military honour, for their bravery.
It emerged this week that Khazzani had watched an Islamist video shortly before attempting the atrocity.
Peter Neumann, director of International Centre for the Study of Radicalisation and Political Violence at King's College London, recently told a London conference: "Wherever I go in Europe and speak to security services, I see that they have been driving in fifth gear for a year or two. The problem is not that they are struggling to understand the problem, it is that they are struggling to cope with number of cases that they have on their desks."
