Britain's top counter-terrorism officer has dismissed suggestions that authorities should allow people to join Islamic State.
Met Police assistant commissioner Mark Rowley said he did not agree with the concept of letting UK citizens travel to territory controlled by the extremist group.
Earlier this month Robert Quick, who was head of special operations for Scotland Yard from 2008-09, questioned whether the Government should "lay on charter flights" for those intent on travelling to a "caliphate" declared by IS rather than allowing them to "fester" in the UK.
Appearing at the parliamentary Home Affairs Committee, Mr Rowley said he did not know the context of the comments - made in an interview with The Guardian - but "generally speaking", he said, he disagreed.
Mr Rowley said: "I think it's the wrong idea. If you have a British citizen why are you going to allow them to go somewhere where you think they are going to take part in murder and all sorts of awful acts?"
Mr Rowley said there was also the issue of people returning from a war zone.
"What about then coming back more trained, more angry, to this country? I think that's completely unacceptable."
Mr Rowley also disclosed that 307 "significant" witnesses to the Tunisia terror attack have been identified by British police.
Those people saw the "really awful events" on the beach in the resort of Sousse, he said.
A total of 38 people, including 30 Britons, were killed in the atrocity.
Mr Rowley said a "handful" of UK officers remain in Tunisia.
The officer warned that the current threat level of severe - indicating an attack on the UK is highly likely - is likely to remain for a long time.
He said the trend for arresting a terrorist suspect every day continues, adding that the rate may be "tipping past that now".
A third of the work involves disrupting significant extremist activity and the rest centres on dealing with individuals "on the edge" of terrorism, he said.
Mr Rowley said around a quarter of terror probes now involve "vulnerable" people, including those with mental health issues.
Asked about three schoolgirls from Bethnal Green who are thought to have travelled to Syria, Mr Rowley said: "If you have been duped and you haven't taken part in terrorism, you've got nothing to fear."