An international manhunt is continuing for Salah Abdeslam, a key suspect linked to the Paris attacks.
A wanted notice was issued for Abdeslam, a 26-year-old Frenchman born in Belgian capital Brussels, with police warning that he is dangerous.
The French authorities missed an opportunity to detain him just hours after the carnage in Paris when he was questioned and released after a car he was in was pulled over near the Belgian border.
Police had Abdeslam in their grasp when they stopped the car carrying him and two other men early on Saturday.
By then, hours had passed since Abdeslam had been identified as the renter of a Volkswagen Polo that carried hostage takers to the Bataclan concert hall which became the scene of a massacre.
Islamic State (IS) has claimed responsibility for the Paris atrocities and French forces struck back with a massive bombardment of the jihadist group's stronghold in Raqqa, Syria.
Abdeslam is one of three brothers suspected of involvement in the Paris attacks, one of whom - reportedly called Ibraham - died in the Bataclan and the other was arrested in Belgium.
British police and spies are working closely with counterparts in France and Belgium to identify and pursue those behind the massacre.
Security has been beefed up in UK cities and ports as Britons were urged to remain vigilant, although the terror threat level has not been changed from the second-highest "severe" rating.
Speaking after leading a meeting of the Government's Cobra emergency committee, Home Secretary Theresa May said the UK authorities were working to find anyone involved in the "barbaric attacks" in the French capital.
David Cameron said the UK's safety and security depended on "degrading and ultimately destroying" the Islamic State (IS) "death cult".
More details have emerged about the IS attack which was the worst terrorist outrage in Europe for more than a decade, leaving at least 129 dead and 350 wounded.
:: Prosecutors believe three teams of terrorists carried out the co-ordinated attacks
:: As many as three of the seven suicide terrorists killed on Friday night were French. Two were Frenchmen living in Brussels
:: Bilal Hadfi has been named in reports as one of the assailants and had is said to have fought with IS in Syria
:: Three Kalashnikov assault rifles were discovered inside the Seat car used in the attacks which was found in the suburb of Montreuil, four miles east of Paris
:: One of the attackers was identified as 29-year-old Frenchman Ismael Mostefai, who had been flagged for links to Islamic radicalism
:: Seven people have been arrested in Belgium and six in France in connection with the killings, including Mostefai's father and brother
:: At least one of the men arrested in Belgium was a French national
:: It has been reported that two of the attackers passed through Europe as refugees, one using a Syrian passport to enter Greece and the other claiming asylum in Serbia
The only British fatality confirmed so far is Nick Alexander, 36, from Colchester, who was selling merchandise for rock group Eagles of Death Metal when their concert at the Bataclan was targeted.
Video has emerged of the moment the terrorists attacked, firing repeatedly at fans as band members fled the stage.
The UK's ambassador in Paris, Peter Ricketts, laid flowers at an impromptu shrine outside the venue today, describing it as "intensely moving".
In a sign of the continued tension in Paris, the Place de la Republique - where huge crowds had gathered - was suddenly evacuated with people fleeing in terror.
The square, where Channel 4 News was broadcasting live at the time, was reopened after it was confirmed the panic was the result of a false alarm.
Speaking from the G20 summit in Turkey, Mr Cameron said Europe would be safer if the threat from IS - also known as Isil - was dealt with.
He said: "It's become even more clear that our safety and security depends on degrading and ultimately destroying Isil whether it's in Iraq or Syria.
"We're playing a huge role in that already in Iraq. Others are taking action in Syria which we both support and enable, but we've got to keep on making the case that we will be safer in the UK, in France, right across Europe if we destroy this death cult once and for all."
It has emerged Iraqi intelligence warned countries in the US-led coalition against IS, including France, of an imminent assault the day before the Paris attacks.
But the Iraqi dispatch provided no details on when or where the attack would take place, and a senior French security official described it as the kind of warning French intelligence gets "all the time" and "every day".
Mrs May said the UK's spies and police were assisting their counterparts on the continent in the hunt for members of the terror cell responsible for the Paris massacre.
She said: "The UK police and security services are working very closely with their counterparts in France and Belgium to identify all those involved and to pursue anyone who may have been involved in the preparation of these barbaric attacks."
She added: "The UK stands shoulder to shoulder in solidarity with France. The terrorists will not win, we will defeat them."
Extra security measures have been put in place around Britain, Mrs May said, with enhanced checks at ports and an increased police presence in cities.
Mrs May confirmed that there were "tried and tested" measures for the military to respond to a marauding attack by terrorist gunmen.
Reports have suggested that special forces are geared to intervene if an attack like the one carried out in Paris was attempted on British soil.
The Home Secretary said: "Since the firearms attack that took place in 2008 in Mumbai, we have been building the capability of police here in the UK to respond to a multiple firearms attack and developing that capability - different training for the police and ensuring that they are able to respond quickly to such an event.
"Indeed there was a major exercise just this summer on the streets of London testing what their response would be to an attack of this sort.
"Of course there are tried and tested arrangements in place for military support to be provided to the police when that's necessary.
"We are obviously reviewing these arrangements to see whether there is anything we need to learn from what has happened in Paris to further develop our capability."
Britons have been urged to join in an Europe-wide minute's silence on Monday at 11am UK time - noon in Paris - while flags have been lowered to half mast over Whitehall buildings