Extremists who are unable to reach the Middle East to fight for so-called Islamic State may carry out attacks in the UK instead, the Director of Public Prosecutions warned as it emerged there have been more than 100 convictions in the UK for Iraq and Syria-related terrorism.
Alison Saunders said the authorities needed to be "very aware" of the risk posed by people who were unable to reach Syria but instead focused on actions in Britain, either plotting attacks or radicalising others.
Analysis by the BBC showed that 85% of the 109 people convicted of terrorism offences related to Iraq or Syria had never set foot in either of the countries.
Ms Saunders told the broadcaster: "We need to be acutely aware that if people can't go to Syria - and we have certainly seen this in some of the cases we have prosecuted - they may plan an attack here instead or they may to more to radicalise other people to attack."
She said "we need to be very aware of that" and it may mean that the "type of prosecutions" seen over the last few years may have to change.
The analysis shows the youngest offender was a then 14-year-old from Blackburn, Lancashire, who was convicted in 2015 after taking on the role of "organiser and adviser" to an alleged Australian jihadist in a plan to murder police officers in Melbourne on Anzac Day.
Those convicted come from a wide cross-section of society and include former prisoners, a hospital director and the son of a police officer, the BBC said.
Married couples, siblings and a mother of six have also been prosecuted and of the 109 people convicted, 18 (16%) were female.
Prisons Minister Sam Gyimah told BBC Radio 4's Today programme that the figures showed the "changing nature of the threat", with "frustrated travellers" plotting or carrying out low-tech "DIY attacks".
"We also know that IS is discouraging people from travelling over to the caliphate to help fight there and is encouraging them to perform jihad locally," he said.
He said the internet was a "key front in the fight against Islamist terrorism" and Ms Saunders acknowledged that prosecutors had to be "on top of" the issue.
Extremists use the internet and social media for communication, spreading propaganda and radicalising others.
Mr Gyimah said Home Secretary Amber Rudd had held meetings with Google and Facebook about the issue.