A source told the Daily Star that people returning to the UK are "tough, possibly traumatised individuals who are effectively addicted to fighting" after surviving months and possibly years of war.
The UK's MI5 reportedly worries that hundreds of Daesh* supporters have set up groups determined on bringing terror to the streets of almost every British city.
"These individuals are not just going to give up the fight — waging jihad is their life. There will be a calm before the storm when they begin to recruit and organise, but the storm is coming", the Daily Stare quote the source as saying.
The terror threat has remained high in the United Kingdom in recent years. The country has experienced a number of bloody terror attacks, most notably the attack on London's Westminster Bridge and parliamentary grounds in March 2017, as well as the Manchester Arena bombing, which claimed the lives of 23 people, including the attacker, on 22 May 2017.
*Daesh (also known as ISIS/ISIL/IS) is a terrorist group banned in Russia