The prime minister said MI5 made “incredibly difficult judgments” on the UK’s behalf.
His comments came after it emerged Mohammed Emwazi was known to authorities.
David Cameron said he would not comment on specific cases but urged the public to back the security services.
Mohammed Emwazi, who is in his mid-20s, first appeared in a video last August, when he apparently killed American journalist James Foley.
He was later thought to have been pictured in the videos of the beheadings of British aid worker David Haines, American journalist Steven Sotloff, British taxi driver Alan Henning, and American aid worker Abdul-Rahman Kassig, also known as Peter Kassig.
UK-based advocacy group Cage has suggested that MI5 may have contributed to the radicalization of Mohammed Emwazi.
Downing Street said the claim was “completely reprehensible”, while London Mayor Boris Johnson described Cage’s comments as “an apology for terror”.
David Cameron defended the security services, praising the work of “these extraordinary men and women”.
He said: “I meet with them regularly, I ask them searching questions about what they do and in my almost five years’ experience as prime minister, I think they are incredibly impressive, hard-working, dedicated, courageous and effective at protecting our country.
“All of the time, they are having to make incredibly difficult judgements and I think basically they make very good judgements on our behalf, and I think whilst 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.”
David Cameron went on to say the security services’ “dedication and work has saved us from plots on the streets of the UK that could have done us immense damage” within the last few months.
He said he was satisfied there was effective scrutiny of the work they do.
Mihammed Emwazi has appeared in videos dressed in a black robe with a black balaclava covering all but his eyes and top of his nose.
Speaking with a British accent, Jihadi John taunted Western powers before holding his knife to the hostages’ necks, appearing to start cutting before the film stopped.
Hostages released by ISIS said Jihadi John was one of three British jihadists guarding Westerners abducted by the group in Syria.
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