A rights group in contact with the man thought to be Islamic State group executioner "Jihadi John" before he left Britain for Syria is facing a backlash after accusing intelligence services of radicalising him.

Cage, whose leading figures include former Guantanamo detainee Moazzam Begg, describes its work as supporting people arrested or raided as a result of the "war on terror" following the 9/11 attacks in 2001.

When Mohammed Emwazi, a Kuwait-born London IT worker, was identified by the Washington Post as "Jihadi John" last week, Cage held a press conference at which its research director Asim Qureshi described him as a "beautiful young man" and blamed British intelligence for radicalising him.

It claimed that MI5 had been tracking Emwazi since at least 2009 and had even tried to recruit him.

Emwazi had complained of harassment between 2009 and 2012, had been stopped from going on a safari trip and prevented from pursuing a job and a marriage in Kuwait, the group said.

The group's comments prompted a furious response across the board in British politics and media, with many arguing that "Jihadi John" -- believed to be responsible for beheading at least five Western hostages -- was the only person accountable for his own actions.

Cage's views were dismissed as "reprehensible" by the office of Prime Minister David Cameron, "very false" by a former head of MI6 and an "apology for terror" by London Mayor Boris Johnson.

"Being turned back on his travels and questioned by the security services had, it seemed, left Emwazi with no alternative but to join Islamic State and behead seven innocent people," wrote Andrew Gilligan in this week's Sunday Telegraph, summing up the views of many commentators.

Amnesty International, which has in the past jointly called for an investigation of British involvement in the CIA's controversial rendition programme with Cage and other bodies, has also spoken out.

"I can't condemn strongly enough anybody in any context who seeks to find some justifications somehow for why you can kill a civilian," Steve Crawshaw, director of the office of the secretary general at Amnesty, told BBC radio.

- 'Legitimate questions' -

At a press conference in London on Thursday following the reporting of Emwazi's name, Qureshi seemed at times choked with emotion.

"There are several young Britons whose lives were not only ruined by security agencies, but who became disenfranchised and turned to violence because of British counter-terrorism policies coupled with long-standing grievances over Western foreign policy," he said in a separate statement.

The same plight had befallen Emwazi and Michael Adebolajo, one of two men who hacked to death 25-year-old British soldier Lee Rigby on a London street in 2013, the group said.

But despite the heat of the backlash against such comments, others defend Cage and its work.

The Joseph Rowntree Charitable Trust, a grants body which gave the group �305,000 ($471,000, 421,000 euros) between 2007 and 2011, was among them.

"We believe that they have played an important role in highlighting the ongoing abuses at Guantanamo Bay and at many other sites around the world, including many instances of torture," the trust said.

"We believe that Cage is asking legitimate questions about security service contact with those who have gone on to commit high-profile and horrific acts of violence."

Cage's work was described as "vital" by the director of human rights group Reprieve, human rights lawyer Clive Stafford Smith.

"I myself represent those said to be 'terrorists' and since Magna Carta, in 1215, we have presumed people innocent rather than guilty," he told the BBC.

"If criticism must be levelled, it should be aimed at those who betray the fundamentals of our legal system by locking people up without trials, or just assassinating people with drones."

Bill Durodie, professor and chair of international relations at the University of Bath, said he believed that Cage's view fed into a wider issue.

"Their accusations of harassment of Mohammed Emwazi by the British security services feed off and into a simplistic narrative of systematic hurt and offence causing unmanageable harm," he said.

Perhaps seeking to make a point after days of pummelling, Cage has now posted a string of messages on its website from people it says have contacted the organisation recently to praise and defend its work.

It has also offered to meet Cameron for a "full and frank discussion" of the issues raised by the "Jihadi John" case.