The government was under mounting pressure on Sunday night to impose tougher penalties on Islamist extremists who travel abroad to fight with Islamic State (Isis).
George Carey, the former archbishop of Canterbury, said jihadists fighting overseas should lose their British passports and be barred from returning to the UK. He also criticised multiculturalism for allowing immigrant communities to establish "separate identities in our cities".
In an unusual alliance, David Davis, the Conservative backbencher and one of parliament's most outspoken libertarians, also said British jihadists should be stripped of their citizenship, even though this might be against international law.
"Lawyers would say you cannot render someone stateless. Perhaps, perhaps not," said Davis. "Whitehall lawyers have been wrong before. Democracies have a right to defend themselves."
In an article in the Telegraph on Saturday, Theresa May, the home secretary, seemed to announce three new proposals that could be adopted to tackle radicalisation: new civil powers (similar to antisocial behaviour orders); widening the use of banning orders against extremist groups; and a new anti-radicalisation duty for public bodies.
May said she already had the power to strip extremists with dual nationality of their citizenship and said the government had recently passed legislation to enable the Home Office to remove citizenship from naturalised Britons. But she said she could not do the same for people born in Britain because "it is illegal for any country to make its citizens stateless".
But in an article in the Mail on Sunday, Davis mocked the measures, saying the government's response to the threat posed by extremists in the UK was "tentative, uncertain, almost limp".
He went on: "Asbos for terrorists? It is hard to imagine the Isis killers quaking in their boots."
Anyone who went abroad to fight for the Soviet Union in the days of the cold war would have been viewed as a traitor, Davis said, but those fighting with Isis were even worse because they were engaged in "industrialised murder".
"Since these young men are in effect swearing allegiance to a hostile state, they should all forfeit their British citizenship – not just those who are dual nationals. Since this is an incredibly serious penalty, it should be done only after a proper public trial carrying all the public seriousness and opprobrium of a murder trial, because in many cases that is what it would be.
"[Isis] is claiming to be a state. They can issue these young men with Islamic State passports if they so wish. It is not our problem that they would have trouble getting into any civilised country with them. Neither will it be our problem any more if [Isis] ceases to exist.
"We must face head-on the paradox that these men can burn their British passports on TV and deny their legal allegiance to Britain, yet our nation cannot say to them, in effect: 'OK, never come back.'
"The result would be that these young men would suddenly find their trip to Syria is no longer a short, violent holiday but a life sentence to the lifestyle they claim to espouse, complete with sharia law and a desert climate."
In a separate article for the paper, Lord Carey said that British jihadists should have their passports withdrawn and that they should be permanently banned from the country.
"Young people who travel abroad to commit violent 'jihad' should know before they go that there is no going back to civilised society," he wrote.
"It may focus their minds to know that the privileges and luxuries of our country (including our gyms, games consoles and relative peacefulness) will be denied to them in future."
The mayor of London Boris Johnson said in an article in today's Daily Telegraph that new laws should be brought in that would mean anyone visiting Iraq and Syria would be automatically presumed to be terrorists unless they had notified the authorities in advance.
There are various international laws preventing governments from making their citizens stateless including the 1954 UN Convention on Stateless Persons and the 1961 UN Convention on the Reduction of Statelessness, both of which the UK has ratified. A Home Office spokeswoman said on Sunday that Davis's idea was a nonstarter. "If someone is born British, you cannot do that," she said.
A government source suggested that Davis was being inconsistent because in May, when the Commons was voting on the legislation about taking away citizenship from naturalised citizens, Davis voted with Labour on one occasion in favour of the proposal being referred to a joint committee. "It's surprising if someone who voted against the government on that now thinks we should render people stateless in defiance of international law," the source said.
But in an interview on the World at One, Davis said that he believed the courts should take the decision to deprive someone of their citizenship, not the home secretary as the recent legislation proposed.
A person born in Britain can lose their nationality if they have citizenship somewhere else and Davis argued that jihadists fighting for Isis were now "effectively swearing allegiance to a proto-state".
Carey was not explicit in his article about whether jihadists should lose their citizenship, or whether they should just lose their passports and be banned from the UK. Britons do not have an automatic right to a passport, and the Home Office has the power to remove passports, but not citizenship, from suspected terrorists, although in practice this power has been used (23 times, in relation to Syria) to stop people leaving the UK, not returning to it.
Carey said there was a need "to recover a confidence in our own nation's values … The fact is that for too long the doctrine of multiculturalism has led to immigrants establishing completely separate communities in our cities. This has led to honour killings, female genital circumcision and the establishment of sharia law in inner-city pockets throughout the UK."
All three of the supposedly new measures announced by May in her Telegraph article on Saturday were actually recommendations in a report from David Cameron's extremism taskforce in December last year. At the time, Cameron said he would "make sure [these measures] are taken forward".
