Lord Macdonald compared the current situation to the US-run detention camp in Guantanamo Bay, which has been used to hold hundreds of terror suspects without trial.
“I think we’re just demonstrating an unwillingness to take responsibility, I think it’s an embarrassment personally … a coward’s form of Guantanamo,” the crossbench peer added.
He was among witnesses giving evidence to the All Party Parliamentary Group (APPG) on Trafficked Britons in Syria on Monday.
The session heard that the UK’s policy was at odds with a growing number of countries, including the US, Germany, Finland and Denmark, who have been repatriating people from camps run by the Syrian Democratic Forces (SDF).
Britain has refused to repatriate British citizens from camps and prisons, where many have been held since early 2019, while using citizenship deprivation and other controversial legal measures to prevent people returning voluntarily.
Ministers have floated the possibility of trials in Iraq or Syria, but Lord Macdonald called the possibility “preposterous” on logistical and international law grounds.
He urged the government to “set our justice system loose” and formally attempt to prosecute suspected Isis members who travelled to Syria from the UK.
“I am confident that many of these individuals would face prosecution because we do know a lot, and many of them have spoken about themselves on social media,” Lord Macdonald added.
“There are other means by which we can place some restraints on people we have to release.”
A former senior official in the US State Department's Bureau of Counterterrorism said repatriating people from Isis camps was “the right thing do to from a security perspective”.
Chris Harnisch, who was deputy coordinator for the department from 2018 until earlier this year, said the Trump administration chose to take back “Americans of all ages”.
He told the APPG that other countries must do the same to “prevent the re-emergence of the caliphate” and that there was “no viable alternative”.
“Isis’ leadership has made clear that the men, women and children in prisons and camps are strategic assets,” he added, warning of repeated attempts at large-scale prison breaks.
“The US and UK and the whole world is at more risk … al-Hol is the capital of the caliphate at this point - you have more hardened adherents to Isis ideology living in that camp than anywhere in the world.”
Mr Harnisch warned that escapees could bolster Isis’ ranks in Syria and Iraq, travel to battlefields elsewhere or return home undetected and plan attacks.
He urged ministers to look at the effect of previous prison breaks that strengthened the Taliban in Afghanistan and contributed to the rise of Isis in 2013.
Mr Harnisch said that Isis camps and prisons were a “hotbed” of radicalisation and terrorist activity, including fundraising and propaganda for Isis, and that indoctrinated children could become a future threat.
He called for the UK to “think twice before stripping nationals of citizenship”, adding: “Such an approach is misguided and will make us all less safe.”
Shahzad Akbar, Pakistan’s federal minister, said his government had not been told in advance of citizenship deprivation plans for terrorists in the past.
He said Pakistan was currently renegotiating its agreement with the UK because of Brexit, and wants any move to make former British or dual nationals its problem to be by “mutual consent”.
“It’s a very self-centred approach to a global problem,” Mr Akbar told the APPG. “It’s a policy of making your problem someone else’s problem.”
He described the creation of separate penalties for naturalised and natural-born British citizens “problematic and racial in nature to say the least”.
Ministers have previously cited safety concerns and logistical issues in Syria as a bar to repatriating Isis suspects, but the APPG was told that other countries had managed.
Mr Harnisch said the US organised for detainees to be transferred to Iraq and then flown back.
Jussi Tanner, a special envoy for Finland’s ministry of foreign affairs, told how he had himself visited Isis camps in Syria and vetted people for repatriation.
“We certainly have had challenges related to security, in terms of how to arrange our visits, but they have not been insurmountable,” he told MPs and peers.
A government spokesperson said: “Our priority is to ensure the safety and security of the UK. Those who remain in Syria include dangerous individuals who chose to stay to fight or otherwise support a group that committed atrocious crimes including butchering and beheading innocent civilians.”
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