Extremists like Mohammed Emwazi are not becoming radicalised because of their contact with security services, the ex-chief of MI6 Sir John Sawers has said.
He told the BBC such suggestions were "very false and very transparent".
Security services have been criticised for being aware of Emwazi, known as "Jihadi John", and not preventing him from joining Islamic State.
But they would face "more criticism" if someone they were not aware of committed an atrocity, he said.
Emwazi, who has been pictured in the videos of the beheadings of Western hostages, was identified this week as a Kuwaiti-born British man, in his mid-20s and from west London.
One of his former teachers said Emwazi had received anger management therapy in his first year of secondary school after "he used to get into fights".
But the teacher, from Quintin Kynaston school in north-west London, said after the therapy the youngster appeared to turn his life around and was a "success story" who went on to achieve everything that he wanted to do.
Speaking to BBC Two's Newsnight, she said other members of staff had been "shocked" to hear about the attacks.
"He was a lovely, lovely boy," she said.
"He had a real willingness to try and succeed."
The teacher, who wanted to remain anonymous, also said MI5 had interviewed his former teachers from the school.
She added: "We'd find that he'd get very angry and worked up and it would take him a long time to calm himself down, so we did a lot of work as a school to help him with his anger and to control his emotions
"It seemed to work. He had a lot of respect for all of the work that had been done for him at our school.
"He didn't come from a troubled background. He didn't leave school with no qualifications. He had every chance of doing well. I just can't believe he'd do that."
Sir John Sawer told BBC Radio 4's Today programme for an interview to be broadcast on Saturday, that extremists "draw attention to themselves because of their activity, their mixing, participation in extremist and sometimes terrorist circles".
He said an approach by security services to individuals they suspected of becoming extremist was an opportunity for them "to draw back from the terrorist groups" and acted as a warning.
"But the idea that somehow being spoken to by a member of MI5 is a radicalising act, I think this is very false and very transparent," he said.
MI5 is coming under pressure to explain why its officers spent five years talking to Emwazi before he left the country for Syria.
'Thousands of concern'
Sir John, who served as head of MI6 for five years, defended the security services, saying they "do a really professional job".
"Of course they know many of these individuals, most of them who end up taking part in terrorist organisations," he said.
"They'd be more subject to criticism if someone came and committed an atrocity in this country or elsewhere who they had no knowledge of whatsoever."
He said: "There are probably several thousand of these individuals of concern and the numbers are rising as more people go to Syria and Iraq.
"And no-one is talking about rounding up all these people or keeping 100% coverage, there's just not the resources to do that and it would be contrary to our principles of human rights to do that, so you have to find a balance."
UK-based advocacy group Cage has suggested that MI5 may have contributed to the radicalisation of Emwazi.
Downing Street said that suggestion was "completely reprehensible", while London mayor Boris Johnson described Cage's comments as "an apology for terror".
Mr Cameron has defended the UK's security services, praising the work of "these extraordinary men and women".
Mr 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.
'Talk a good game'
Mr Johnson said it did renew the argument about control orders - a form of house arrest for terrorist suspects - which were abolished by the coalition government in 2012 and replaced with Terrorism Prevention and Investigation Measures (TPims).
"It is vital when you are controlling these people to be able to relocate them, to take them away from their support networks and to monitor them properly," he said.
David Anderson, the government's independent reviewer of terror legislation, said it was difficult not to have sympathy with the security services "when you see just how many cases they have to look at".
"A lot of people talk a good game when it comes to terrorism. The knack is identifying the few who are going to do something about it."
Mohammed Emwazi's movements before heading to Syria
- 1. Aug 2009, refused entry to Tanzania: travels to Tanzania with two friends, but is refused entry at Dar es Salaam. Tanzanian police have denied Emwazi's name is on their database of suspected foreign criminals detained and deported in 2009, as he had claimed. Emwazi and his friends are put on flight to Amsterdam, where they are questioned. They return to Dover and are questioned again.
- 2. Sept 2009, travels to Kuwait for work: leaves the UK for Kuwait for work.
- 3. May/June 2010, returns to UK for holiday: he returns to the UK for an eight-day visit.
- 4. July 2010, refused re-entry to Kuwait: Emwazi returns to the UK once more for a couple of days. He is stopped at Heathrow on his return to Kuwait and told he cannot travel as his visa has expired.
- 5. 2013, travels to Syria: Emwazi changes his name to Mohammed al-Ayan and attempts to travel to Kuwait but is stopped and questioned. Three days later, he heads abroad. Police later inform his family he has travelled to Syria.