Ed Miliband has proposed radical action to stop young Britons being drawn to extremist ideology, in a stark warning that tougher measures are needed to prevent would-be jihadis travelling to join Islamic State (IS) militants in Iraq and Syria.
The Labour leader has suggested a mandatory programme of de-radicalisation for those involved on the fringes with IS and called for ministers to revisit the decision to scrap the control orders regime for terror suspects.
His comments came as David Cameron prepared to set out the steps he would take to tackle the threat posed by IS extremists after experts warned that a terror attack on the UK was "highly likely".
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The Prime Minister will make a statement in the Commons on Monday, spelling out the extra powers after acknowledging there were "gaps in our armoury" which needed to be filled.
Writing in The Independent, Mr Miliband stressed the need for a "multilateral alliance" combining political, diplomatic and humanitarian action to counter the "perverted mission" of IS in the Middle East.
In the UK, he said, "we should act with clarity and determination" to tackle the threat from extremists.
"Such action must include an overhaul of the Home Office Prevent programme to equip communities with the tools to stop young people being sucked into extremist ideology," he said.
"We should also do more to prevent potential fighters from travelling to the region, and promote an effective, mandatory programme of de-radicalisation for anyone who is drawn into the fringes of extremism in Syria and Iraq."
He highlighted concerns raised by David Anderson, the independent reviewer of counter-terrorism legislation, about the terrorism prevention and investigation measures (Tpims).
Mr Anderson has recommended stronger measures to restrict the movements of people subject to Tpims, including powers similar to those under the scrapped control order regime allowing the authorities to relocate a terror suspect to another area.
Mr Miliband said: "For the most serious and high-risk cases, as the independent reviewer on terrorism has recommended, the Government should strengthen existing powers, including revisiting the case for control orders."
At a Downing Street press conference Mr Cameron insisted there would be no "knee-jerk response" to the events in the Middle East, and insisted people should continue to "go about our daily lives in our normal way".
But as the terrorist threat to the UK was raised from substantial to severe, the Prime Minister warned that extremism in Iraq and Syria posed a greater danger to Britain than al Qaida.
Mr Cameron, police and intelligence officials have been concerned for months about the threat to the UK posed by British jihadis returning to the UK after travelling to Syria or Iraq.
During the press conference, he highlighted the danger of extremists returning to Britain and said the "scale of this threat is growing".
"I said very clearly last week that there would be no knee-jerk response," he said. "But we have to listen carefully to the security and intelligence officers who do so much every day to keep us safe.
"It is becoming clear that there are some gaps in our armoury, and we need to strengthen them.
"We need to do more to stop people travelling, to stop those who do go from returning, and to deal decisively with those who are already here.
"I will be making a statement in the House of Commons on Monday.
"This will include further steps to stop people travelling, with new legislation that will make it easier to take people's passports away."
The Prime Minister said the gruesome murder of US journalist James Foley, apparently by a British jihadi, was "clear evidence - not that any more was needed - that this is not some far off (problem), thousands of miles away, that we can ignore".
The threat level was raised from substantial to severe by the Joint Terrorism Analysis Centre (JTAC) due to fears about British jihadists returning from Iraq and Syria.
The change means a terrorist attack is considered "highly likely", although Home Secretary Theresa May stressed there was no specific intelligence.
"The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West," Mrs May said. "Some of those plots are likely to involve foreign fighters who have travelled there from the UK and Europe to take part in those conflicts."
Speculation is mounting that the Government could bow to pressure for Tpims to be beefed up. There have been calls for powers to impose "internal exile" on suspects, a key part of the old control orders regime, to be restored.
Mr Cameron is to push for more co-ordinated European action to track jihadists at a summit in Brussels.
The UK wants to revive a directive that would enable police and security services across the EU to share passenger records.
National leaders have signed off the arrangements - but they have stalled in the European Parliament after MEPs expressed concern about civil liberties and privacy.