British police are carrying out security investigations at an "exceptionally high" pace not seen in years to disrupt potentially deadly plots directed from abroad, notably Syria, the country's top counter-terrorism officer said on Friday.
Mark Rowley, Britain's national policing spokesman for counter-terrorism, said the police had made 218 arrests so far this year while dozens of vulnerable people were also being referred to de-radicalisation programmes.
"The volume, range and pace of counter-terrorism activity has undergone a step-change," Rowley said in a statement, in the latest indication of serious concern among British authorities about the threat of an attack.
Britain has been considered a target for Islamist militants ever since the Sept. 11, 2001, al Qaeda attacks on the United States, and 52 people were killed when four young Britons carried out suicide bombings in London in 2005.
In August, Britain raised its threat level to the second-highest classification of "severe", meaning an attack was considered highly likely, for the first time since mid-2011.
Prime Minister David Cameron has warned that Islamic State (IS) militants fighting in Syria and Iraq posed the country's greatest ever security risk and has promised further measures to counter a threat posed by returning veterans. Some 500 Britons are already thought to be fighting there.
Rowley's is the latest in a string of warnings from security officials and senior politicians.
Last week, he issued a notice to police officers around the country to be on alert to possible risks to their safety, while London Mayor Boris Johnson has said spies are monitoring thousands of suspects.
Just this month, detectives have arrested 14 people in operations, and one very senior officer speaking on condition of anonymity has told Reuters some suspected plotters had got close to carrying out an attack.