The battle-hardened militants who had left Britain to fight with the Takfiri group had been expected to return home as the terrorist group continued to lose ground, officials said.
They noted that it is impossible for security forces to keep watch on all the returning fanatics due to the sheer scale of the arrivals.
The disclosures come amid increased security operations after the recent attack outside the British parliament when an assailant plowed a car into pedestrians and stabbed a police officer leaving at least four people dead.
British authorities say that at least 800 nationals have traveled to Syria and Iraq to fight alongside various Takfiri groups, regarding them as potential security threats upon returning to the country.
Last February, the European Union's criminal intelligence agency Europol said up to 5,000 trained members of Daesh were at large in Europe, adding that some 30,000 militants from over 100 countries had reportedly traveled to Syria and Iraq since 2011 to join the ranks of Takfiri terrorist groups.
Daesh terrorists are mainly operating in Syria and its eastern neighbor, Iraq.
The US and European countries are also believed to be the main suppliers of arms and other assistance to the terrorists, effectively feeding terror that is returning to affect themselves.