David Cameron says Britain will do "everything we can" to bring terrorists to justice after Londoner Mohammed Emwazi was identified as the Islamic State militant known as "Jihadi John".
The Prime Minister also expressed his support for Britain's security services as questions were raised about whether appropriate actions were taken to stop Emwazi from travelling to Syria.
Speaking at an event in Cardiff, Mr Cameron defended MI5 and praised the service for its "dedicated and courageous" work protecting Britain.
"They are having to make incredibly difficult judgements, and I think basically they make very good judgements on our behalf," he said.
"I think while we are in the middle of this vast effort to make sure British citizens are safe, the most important thing is to get behind them."
The security agency has been criticised for its handling of Emwazi's case. The 27-year-old travelled to Syria in 2013, several years after MI5's first contact with the militant.
In 2009 he was reportedly questioned by an MI5 officer in Amsterdam who accused him of attempting to travel to Somalia to join a terror group.
Asim Qureshi, a director of the CAGE campaign group, has claimed Emwazi was harassed by the security agency over the following years as agents tried to recruit him as an informant.
Sir Menzies Campbell, a member of the Intelligence and Security Committee, has indicated that the parliamentary committee is likely to seek answers from MI5 over what information they held on "Jihadi John".
The families of Western hostages killed by Islamic State have expressed mixed reactions to the identification of Emwazi.
A spokesman for relatives of Steven Sotloff, the US journalist beheaded in an Islamic State video, says they have "full faith" that Emwazi will eventually face justice.
While refusing to address specific cases, Mr Cameron said the security services and the police would do "everything" to bring those who commit "appalling and heinous crimes" to justice.
He described Britain's security services as "incredibly impressive, hard-working and dedicated".
"I'm satisfied we have in place the right ways of scrutinising the work that these extraordinary men and women do on our behalf," Mr Cameron said.
"Even in the last few months their dedication and work has saved us from plots on the streets of the United Kingdom that could have done immense damage.
"So I think it is a moment to stand up and thank them for the work they do on our behalf."
Speaking to Sky News, the executive director of the Human Security Centre, an independent foreign policy think tank, criticised CAGE for suggesting that Emwazi was radicalised as a consequence of his contact with MI5.
"This narrative was put forward by CAGE, which is often described as a human rights organisation, or a civil liberty organisation, but which is actually a pro-Islamist organisation," she said.
"They have a long record of supporting terrorists and of apologising for their actions."
Meanwhile Boris Johnson has told Sky News of his concern about youth being "sucked into" Islamist ideologies in London and across the country.
Mr Johnson was speaking to First News readers and contributors from Sky News' Stand Up Be Counted campaign.
Asked whether people should feel safe in Britain, Mr Johnson said: "We should feel safe in this country. (The) police are doing a great job... and we're a safe society.
"But... we do have a problem with terrorism, with young people who are just being driven into an ideology that in my view is leading them into dangerous and criminal, catastrophic behaviour."
