Britain should encourage jihadis fighting in Syria and Iraq to "come home", the former global counter-terrorism director of MI6 has said.

David Cameron outlined new powers last week for police to seize the passports of terrorist suspects and stop British extremists from returning to the UK. Others, including Boris Johnson, the London mayor, have called for British jihadis to have their citizenship removed.

However, Richard Barrett, a former counter-terrorism chief at MI5 and MI6, said repentant fighters needed "to know that there is a place for them back at home".

His comments follow reports that dozens of disillusioned British jihadis are looking at ways to return to the UK, but fear being imprisoned. Barrett, who also led the UN mission to track down Osama bin Laden, said returning fighters could prove an invaluable asset in dissuading potential jihadis from travelling to fight with Islamic State (Isis).

"Many of the people who have been most successful in undermining the terrorist narrative are themselves ex-extremists," said Barrett, adding that such people can "explain why going abroad to fight is a very bad idea".

He said: "It would seem sensible to encourage British and other foreign fighters who have joined the Islamic State or other extremist groups in Syria and Iraq, and now realise this was wrong, to come home.

"These are the people who can expose the true nature of the Islamic State and its leadership. Their stories of brutality and the motives behind it will be far more credible and persuasive than the rhetoric of men in suits.

"These repentant fighters need a way out, and although the law must take its course, they need to know there is a place for them back at home if they are committed to a non-violent future."

Researchers at the International Centre for the Study of Radicalisation at King's College London have recently been contacted by jihadis who say that they are despondent about the situation in Syria and that, although they fear lengthy custodial sentences, they would be happy to enrol on de-radicalisation programmes.

"These ex-fighters could help the authorities to understand better than they do now why people are still going to Syria and Iraq and what needs to be done to slow the flow to a trickle or stop it altogether," said Barrett.

Pressure for a fundamental reappraisal of Britain's de-radicalisation programme is mounting among experts and politicians. Hazel Blears, a former Home Office minister, echoed the need to use the experiences of extremists to prevent other Britons following in their footsteps. "Provided you start from a sceptical point of view and you can find some who are genuinely remorseful, for whom it's been a terrible experience, and who are prepared to join a programme talking to other young people, then absolutely you should be trying to work with them. It's a very powerful narrative."

Senior Liberal Democrat figures are also understood to be open to the notion of offering jihadis a way back home, so long as they renounce violence. Sir Menzies Campbell, a former party leader, said: "I don't think we could give them a total amnesty, but we could treat them leniently in return for completing a de-radicalisation programme."

The threat posed by Isis will be the subject of a day-long Commons debate on Wednesday. Conservative whips have begun to take soundings at Westminster over Tory MPs' attitude to military action, indicating that the backbench mood was "hardening".

However, Blears added: "There's no point taking military action if you continue to have a supply chain of people ready to go here in Britain and across the world."

More than 500 British citizens are believed to have travelled to the region since 2011. In today's Observer, the controversial British Islamist Anjem Choudary acknowledges Abu Bakr al-Baghdadi, the leader of the self-declared Islamic State, as "the caliph of all Muslims and the prince of the believers".

His support of Baghdadi comes days after a video was released showing the beheading of the American journalist Steven Sotloff, apparently by the same British jihadi member of Isis who had taken part in the decapitation of Sotloff's fellow hostage James Foley.

It also follows a succession of atrocities perpetrated and disseminated by Isis fighters, including massacres of captured Syrian soldiers and reported genocidal attacks on the Yazidi minority. Human Rights Watch says it has identified sites of Isis mass killings in the Iraqi city of Tikrit.

Choudary, who has had links with a number of Muslims convicted under UK anti-terror laws, dismissed the allegations against Isis as propaganda, expressing his admiration for the jihadi state and claiming that Christians were voluntarily returning to Isis-ruled Mosul because they wanted to live under sharia law.

Meanwhile, a French journalist held hostage for months by Isis in Syria has revealed that one of his abductors was Mehdi Nemmouche, the Frenchman suspected of killing four people, two of them Israeli citizens, at the Brussels Jewish Museum this year. French magazine Le Point quoted its reporter, Nicolas Henin, saying that he had been tortured by Nemmouche, who is now in custody. Henin was held for a time with Foley and Sotloff and was released in April.