David Cameron and Nick Clegg will embark on intensive negotiations over the weekend to try to agree a fresh round of measures to tackle the threat posed by terror suspects in Britain.
The prime minister announced that he would unveil new measures in a statement to parliament on Monday at a televised press conference, on the day that the terror threat to the UK was raised from substantial to severe.
Speaking at Downing Street after his return from a pre-referendum trip to Scotland, Cameron said: "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."
He was speaking amid fears that extremists aligned to Islamic State (Isis) plan to return from the conflict-torn region to mount attacks in western Europe. Cameron said that Isis and other extremists posed a "generational struggle" for the UK and other western countries.
Cameron highlighted plans to make it easier to revoke passports for British citizens amid concerns that this power has only been used on 23 occasions in the last year while up to 250 jihadis are thought to have returned to Britain from Iraq and Syria.
It is understood that ministers will embed this power in legislation after it was introduced in April last year through royal prerogative executive powers. The move is designed to make it easier to prevent suspects from travelling to Syria and Iraq. There are separate powers to strip citizenship from UK citizens with dual citizenship.
Cameron did not provide more details of his plans. Government sources later confirmed that he and Clegg will have further discussions over the weekend to reach agreement.
It is understood that one key area, where Cameron and his deputy have yet to reach agreement, is on terrorism prevention and investigation measures (TPIMs), which replaced control orders. Cameron and Clegg are looking closer at the recommendation by David Anderson, the independent reviewer of terrorism legislation, who called for two changes in his annual report in March.
Anderson recommended a strengthening of "locational constraints" on those subject to TPIMs to ban them from some areas or to restore the power to relocate them to specific areas. He also called for powers to force those subject to TPIMs to attend probation service meetings.
Cameron said the beheading of James Foley showed the need to act, though the decision to raise the terror threat alert was not related to the video that featured his killing.
He said: "It was clear evidence, not that any more was needed, that this is not some foreign conflict thousands of miles from home that we can hope to ignore. The ambition to create an extremist caliphate in the heart of Iraq and Syria is a threat to our own security here in the UK."
He outlined the changes after the home secretary, Theresa May, announced the raising of the terror threat level. May said that raising it from substantial to severe meant that an attack was highly likely, though she said there was no evidence to suggest an attack was imminent.
She said the decision was "related to developments in Syria and Iraq, where terrorist groups are planning attacks against the west". She said some of the plots were likely to involve foreign fighters who had travelled to the Middle East from Britain and Europe to take part in the conflicts there.
The threat level last stood at severe more than three years ago and it was reduced to substantial on 11 July 2011. Severe is the fourth level in the five-tier system of alert categories that have been used by the government since 2006 to warn of terrorist activity. The higher level is critical, which is used when an attack is expected imminently.
May said the decision to raise the threat level had been taken by the Joint Terrorism Analysis Centre. "We face a real and serious threat in the UK from international terrorism. I would urge the public to remain vigilant and to report any suspicious activity to the police," she said.
Downing Street knew before the defection of the Tory MP Douglas Carswell to Ukip on Thursday that the terror level threat would be raised. It made preparations for Cameron's statement before the Carswell announcement, indicating that the press conference was not an attempt to deflect attention from Tory divisions on Europe.
Cameron issued a dire warning of the threat posed by extremists at home and abroad as he spoke of the need to tackle support for violent and extremist ideology. He said: "In Afghanistan the Taliban were prepared to play host to al Qaida, the terrorist organisation.With IS we are facing a terrorist organisation not being hosted in a country but seeking to establish and then violently expand its own terrorist state.
"With designs on expanding to Jordan, Lebanon, right up to the Turkish border, we could be facing a terrorist state on the shores of the Mediterranean and bordering a Nato member."
The prime minister made clear that he would be relentless in tackling supporters of extremist ideology. He said: "The ambition to create an extremist caliphate in the heart of Iraq and Syria is a threat to our own security here in the UK.
"The terrorist threat was not created by the Iraq war 10 years ago. it existed even before the horrific attacks on 9/11, themselves some time before the war.
"This threat cannot be solved simply by dealing with perceived grievances over Western foreign policy. Nor can it be dealt with by addressing poverty, dictatorship or instability in the region - as important as these things are.
"The root cause of this threat to our security is quite clear. It is a poisonous ideology of Islamist extremism that is condemned by all faiths and faith leaders."
Yvette Cooper, the shadow home secretary, said that Labour would work with the government to strengthen TPIMs. She said: "The prime minister is right to say this threat is based on a poisonous ideology of warped Islamic extremism that is condemned by all faiths and all faith leaders.
"The head of MI5 warned last year that the threat from Islamist extremism was becoming 'more diffuse, more complicated, more unpredictable', as so many Britons were joining the conflict. I have said to the Home Secretary that we stand ready to work with the Government on any measures that may be needed to respond to the threat."
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