Deputy Prime Minister Nick Clegg has rejected calls for tougher measures to combat the threat posed by British jihadists returning from Iraq and Syria, saying that a new law would not remove the danger from Islamic State (IS) extremists.
Boris Johnson said he wants the reportedly English jihadist who beheaded an American journalist to be killed in a bomb attack, and joined the growing calls for Britons fighting abroad to be stripped of their citizenship.
But Mr Clegg said he would listen to what the police and intelligence services said they needed to tackle the "hateful ideology" of IS rather than accepting the Mayor of London's proposals.
Speaking during a visit to India, he said: " I think the issue is incredibly important, that we should make sure that those young men and it is invariably young men, who are attracted to this hateful ideology that draws them to the bloodshed and conflict in places like Syria and Iraq, they shouldn't be able to come back and do harm on the streets of British towns, villages and cities, and that is now our number one priority.
"It's what the police, the authorities and security services are working flat out on; I have huge confidence in the outstanding work they do.
"With the greatest respect to Boris Johnson, I will listen to what they tell me they need rather than what he says.
"We actually have a number of measures already on the statute book which allow us to keep a very close eye on those people who aren't in prison, aren't sentenced, but nonetheless are perceived to be a threat to the United Kingdom.
"And of course, we will continue to review all the powers on the recommendation of the police and security services that may be deemed to be necessary to deal with this very serious issue."
He added: "I sometimes wish it was as simple as Boris Johnson implies: all we need to do is pass a law and everything will be well."
Intelligence agencies are close to identifying the brutal killer of US journalist James Foley, believed to be a British extremist who has been dubbed "jihadi John".
Mr Johnson said Britain must take on IS and "try to close it down now", warning that doing nothing would mean a "tide of terror will eventually lap at our own front door".
The mayor, who has overall responsibility for the Metropolitan Police, called for new laws that would mean anyone visiting Iraq and Syria would be automatically presumed to be terrorists unless they had notified the authorities in advance.
Writing in his Daily Telegraph column, Mr Johnson said: " Young men such as this killer are famously told that if they die in 'battle' they will be welcomed in heaven by the sexual ministrations of 72 virgins.
"Many of them believe it - even though scholars have suggested that the reference to 'black-eyed virgins' is in fact a promise of 72 raisins. I suspect most of us don't give a monkey's what happens to this prat in heaven, whether he meets virgins or raisins - we just want someone to come along with a bunker buster and effect an introduction as fast as possible."
Mr Johnson said those who "continue to give allegiance to a terrorist state" should lose their British status citizenship and called for swift changes to the law so there is a "rebuttable presumption" that those visiting war areas without notifying the authorities have done so for a terrorist purpose.
The mayor said that while Britain's recent military interventions have left the nation reluctant to wade into overseas conflicts " doing nothing is surely the worst of all" and warned that the IS "wackos" must be tackled.
" What is the point of having a defence budget, if we don't at least try to prevent the establishment of a terrorist 'caliphate' that is profoundly hostile to civilised values?"
But former attorney general Dominic Grieve said Mr Johnson's suggestion that those travelling to Iraq and Syria should be presumed guilty was "draconian".
And h e warned that calls to render suspects stateless by stripping them of British citizenship would mean tearing up a United Nations convention and could have a potentially damaging effect on the UK.
"It's entirely contrary to a United Nations convention of which we are signatories. The reason we signed up to it is because statelessness is a very real problem around the world," he told BBC Radio 4's World at One.
"I think the UK took the view that curbing statelessness was very important because, of course, if you render somebody stateless then another country may do the same thing to you in respect of its own nationals who happen to be situated in your country, leaving you with a major problem.
"So if we are about to rip up a UN convention, we need to think through the consequences."
Liberty director S hami Chakrabarti said: "When Mr Johnson was seeking the vote in London, he spoke passionately against identity cards and punishment without trial.
"Now his political ambitions have escalated,it seems that liberty is trivial and suspicions should replace charges, evidence and proof. Stripping Britons of their nationality is unjust to the innocent and internationally irresponsible if they are guilty."
Shadow Home Office minister David Hanson repeated Labour's call for the return of the control order regime, which was abolished by Theresa May in favour of Terrorism Prevention and Investigation Measures (Tpims).
He said: "Labour opposed Theresa May's decision to end control orders and warned that Tpims would not be effective enough for serious cases. The Home Secretary must now rethink her decision and bring back stronger powers.
"Because of the weakening of the powers, two terror suspects absconded - one in a black cab, one in a burka. Both the independent reviewer and a committee of Parliament has warned that the police and security services have stopped using them because they aren't effective enough.
"And the independent reviewer has said the powers aren't strong enough and new measures should be added.
"The police and security services are warning that there are British citizens who pose a serious threat but who they currently can't prosecute for crimes abroad - these are exactly the circumstances where control orders with proper judicial oversight and safeguards should be considered.
"Theresa May needs to listen to the concern being expressed across the political divide and reverse her decision. She should strengthen the powers either by reintroducing control orders or bringing in the stronger measures recommended by David Anderson, the independent reviewer.
"And she should urgently overhaul the Prevent programme so that more is done working with communities to prevent radicalisation in the first place."
Former director of public prosecutions Lord Macdonald of River Glaven also rejected Mr Johnson's call for an end to the presumption of innocence.
He said: " The issue here is not the inadequacy of our law, our law isn't inadequate, it's the question of detection and capture."
The Lib Dem peer told BBC Radio 4's PM programme: " We have laws in place and there's no doubt that many of these young sociopaths boasting as they are on social media about their grotesque behaviour in that part of the world are creating the very evidence that can convict them when they return to the UK, if they survive long enough to return.
"But when they return of course they should be arrested and put on trial and we have the mechanisms and the wherewithal to achieve that."
He also warned against stripping people of their citizenship: "We need to really work in co-operation with other countries to fight these sorts of threats, not go off in a hissy fit of our own. I don't think it helps to make these people stateless.
"These people need to be either killed in conflict, which is going to happen to many of them, or they need to be captured, put on trial and sent to prison."
Richard Barrett, former global counter-terrorism director of MI6, told The Guardian: "This fundamental tenet of British justice should not be changed even in a minor way for this unproven threat - and it is an unproven threat at the moment."
He said the Government needed to understand better the domestic threat posed by IS before introducing new laws.
"I don't think we should change the laws without a very much more thorough assessment and understanding of the threat," he said.
"Sure, there's a problem with people who go to Syria and they may have broken the law if they joined organisations like Islamic State and al-Nusra Front, but there should be some sort of effort to prove that, rather than assume they've done so."