A wanted notice has been issued by French police hunting for a man suspected to be on the run after the Paris attacks.
The manhunt has been launched for Salah Abdeslam, a 26-year-old born in Belgian capital Brussels, warning people that he is dangerous and saying "do not intervene yourself".
Police said earlier that a search was under way for a French man believed to have been directly involved in Friday's attacks.
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.
The man sought by the authorities is believed to have rented the black Volkswagen Polo used by a group of hostage-takers that left at least 89 people dead inside the Bataclan concert hall.
Speaking after leading a meeting of the Government's Cobra emergency committee, Home Secretary Theresa May said the UK authorities were working to help find anyone involved in the "barbaric attacks" in the French capital.
Britons have been urged to join in an Europe-wide minute's silence on Monday at 11am UK time - noon in Paris.
More details have emerged about the Islamic State (IS) attack which was the worst terrorist outrage in Europe for more than a decade, leaving at least 129 dead and 350 wounded.
:: Seven people were detained in Belgium in connection with the attacks.
:: Two of the terrorists killed on Friday night were Frenchmen living in the Belgian capital Brussels.
:: As many as three of the seven suicide bombers were French, as was at least one of the men arrested in Belgium.
:: Three Kalashnikov assault rifles were discovered inside a 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
:: Mostefai's father and brother have been arrested.
:: Prosecutors believe three teams of terrorists carried out the co-ordinated attacks.
:: At least one of the attackers is believed to have passed through Greece as a refugee.
Nick Alexander, 36, from Colchester, the only British fatality confirmed so far, was selling merchandise for rock group Eagles of Death Metal when their concert at the Bataclan venue 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".
Setting out the British response, Mrs May 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.
"People will see increased security at the borders, increased checks taking place.
"There is also some increased police presence in major cities as well."
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."
A team from the Foreign Office has travelled to Paris to assist Britons injured or traumatised by the violence.
Mrs May said: "There is considerable consular work being done by the British embassy but also a crisis team was sent out from the Foreign Office here in the UK to provide extra support to those who are in hospital, those who have obviously been caught up in this.
"Support will also be available to those who are returning who have been part of witnessing these terrible events."