In contrast, Max Hill QC, the Independent Reviewer of Terrorism Legislation, has said Britons who joined IS through "naivety" should be spared prosecution and instead be reintegrated into society if they return home.
Britain's new Defence Secretary has unequivocally threatened to kill Britons who leave the country to fight for ISIS.
A dead terrorist can't cause any harm to Britain.
The minister also told the newspaper that jihadist groups in Libya, Iraq and Syria were believed to be plotting attacks on the United Kingdom, and that "our job in terms of eliminating will not stop this year, will not stop next year - it is something we have got to continue to pursue".
"I do not believe that any terrorist, whether they come from this country or any other, should ever be allowed back into this country".
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Williamson's comments were criticised by former Liberal Democrat leader Paddy Ashdown, who said defence secretaries "should be measured, careful, judicious and thoughtful about what they say, not apprentice cadet versions of Donald Trump".
And, just weeks ago, Home Office proposals - called Operation Constraint - to de-radicalise returning IS fighters, by giving them council houses and other support, were leaked.
"We have got to make sure that as (they) splinter and as they disperse across Iraq and Syria and other areas, we continue to hunt them down", he said.
And Rory Stewart, the minister for worldwide development, said the "only way" to deal with British IS fighters in Syria is "in nearly every case" to kill them.
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