Hundreds of British extremists could return to the UK if jihadist groups start to lose battles in Iraq and Syria, the country's most senior police officer has warned.
Scotland Yard chief Sir Bernard Hogan-Howe said it cannot be predicted when potential terrorists might decide to return home, and police are preparing in case of an influx.
It is estimated that around 500 Britons have travelled to Syria to fight alongside extremists there, of whom around half are thought to be from London.
Sir Bernard said there are "a significant number of people who run the risk of being radicalised, militarised and potentially desensitised to violence, who one day we assume will come home".
He added: "The biggest concern certainly I have is that none of us know which way those wars are going to go. We've seen some quite radical movements across whole countries. Look at Iraq, the shift in a few days. These world events were moving at pace.
"That was obviously Islamists who were progressing positively. If there were to be a reverse of that at pace, and they were to lose, there is a risk that hundreds of people would want to come home. That's the biggest challenge.
"We can't predict when it might happen. Should there be large numbers returning it puts great pressure on all of us to make sure that we are kept safe.
"While they are there they are not an immediate threat, but should they come back, and should they come back together, that's a concern. We are trying our best to prepare for that. No one should underestimate the task."
A series of arrests have been made in recent months of suspects either travelling back from war-torn Syria, trying to reach the country from the UK or attempting to send funds for terrorism there.
The typical route for would-be jihadis is to fly to Turkey and then travel overland to Syria.
UK authorities have warned of the dangers of going to the Middle Eastern state, with a controversial campaign aimed at encouraging female relatives of aspiring terrorists to contact police.