More than 700 Britons are thought to have traveled to Syria and over a half have since returned home where they now pose a significant terrorism threat, British police said on Thursday.
Mark Rowley, the lead officer on counter-terrorism, also said there had been a record 338 arrests for terrorism-related offences last year, up by a third from 2013, with almost a half related to the conflict in Syria.
Rowley said of the 700 Britons who were believed to have gone, "a significant proportion" were thought to have tried to join Islamic State (IS) militants who have taken over swathes of Syria and Iraq.
About a half of those who had gone to the region were now thought to be back in Britain.
"[IS] and other terrorist groups are trying to direct attacks in the UK; encouraging British citizens to travel to Syria to fight and train; and are seeking, through propaganda, to provoke individuals in the UK to carry out violent attacks here," Rowley said in a statement.
He said the threat ranged from "lone actors intent on carrying out crude and violent attacks" to more sophisticated and ambitious coordinated plots by organized networks. Of those arrested last year, 11 percent were women and 17 percent were aged under 20.
Last August, Britain raised its terrorism threat alert to the second-highest level of "severe", meaning an attack was considered highly likely with security chiefs repeatedly warning about the danger posed by battle-hardened veterans of fighting in Syria returning home.
Among those Britons who have joined the conflict is Kuwaiti-born Londoner Mohammed Emwazi, dubbed "Jihadi John" by media, who has appeared in several Islamic State videos showing the beheading of Western hostages.
The authorities say they have thwarted major plots every year since four young British Islamists killed 52 people in suicide bomb attacks on London's transport network in 2005.
In February, a British teenager was convicted of plotting to behead a soldier after becoming influenced by the brutal killing of Afghan war veteran Lee Rigby by two Muslim converts on a London street in 2013, the last deadly Islamist attack in Britain.
The police figures come a day after Prime Minister David Cameron announced plans for new laws to curb organizations and individuals who promote militant ideologies at home and recruit young people to radical Islamist groups abroad.