A "handful" of Britons are feared to be among the dead after gunmen and bombers rampaged across Paris, carrying out the worst terrorist attack in Europe for more than a decade.
The first British national was confirmed dead as details began to emerge of the Islamic State (IS) terror cell responsible for the marauding attack in the French capital.
But a UK Government source said the picture remained unclear and indicated the British death toll was expected to rise.
"We know of one death already, we fear there may be a handful of British fatalities and about the same number are being treated for their injuries in hospital," the source said.
The number of those killed in the series of attacks in Paris was revised up to at least 129, with 352 people injured, 99 critically.
David Cameron earlier warned that the UK should "be prepared for a number of British casualties", as he told the French people: "Your fight is our fight."
During the rampage, scores were killed at the Bataclan concert hall, two suicide attacks and a bombing took place at the Stade de France stadium where French president Francois Hollande was among thousands of football fans watching the national side play a friendly against Germany, and gunmen targeted bars and restaurants in the 10th and 11th arrondissements of central Paris.
Details about the terror cell which carried out the attack have started to emerge as authorities across Europe carried out investigations.
:: Prosecutors believe three teams of terrorists carried out coordinated attacks.
:: All seven suicide attackers wore identical explosives belts.
:: One of the suicide bombers at the Bataclan has been identified as a young Frenchman flagged in the past for links with Islamic extremist activity.
:: Suicide bombers who targeted the Stade de France were found to have Egyptian and Syrian passports.
:: One was reported to have had a ticket for the game and detonated his bomb after being stopped by security staff trying to enter the stadium.
:: The authorities in Athens said the holder of the Syrian passport had passed through Greece as a refugee.
:: Three arrests linked to the deadly attacks in Paris were made by the authorities in Belgium, after a car with Belgian license plates was seen close to the Bataclan,
European leaders have called for a minute's silence at noon on Monday in honour of those killed in the carnage in the French capital.
Mr Cameron chaired a meeting of the Government's emergency committee and also spoke to Mr Hollande following the atrocities.
The Prime Minister told Mr Hollande "that the UK stood with France and the French people during this difficult time and we would do all we can to help".
The two leaders agreed to "further enhance" co-operation and information-sharing between the UK and France "to ensure we are doing all we can to identify and stop those who threaten us, whether in Syria and Iraq or closer to home".
Mr Cameron said the terror threat level in the UK would remain at "severe" but the Paris attack would prompt a review of plans and suggested the threat posed by Islamic State was "evolving".
The Prime Minister said: "The events in Paris are the worst acts of violence in France since the Second World War, the worst terrorist attack in Europe for a decade, a horrifying and sickening attack.
"Our hearts go out to the French people and to all those who lost loved ones.
"Today the British and French peoples stand together as we have so often before in our history when confronted by evil."
He added: "These were innocent victims enjoying a Friday night out with friends and family, no doubt at the end of a hard week. They were not seeking to harm anyone, they were simply going about their way of life - our way of life.
"They were killed and injured by brutal and callous murderers who want to destroy everything our two countries stand for: peace, tolerance, liberty. But we will not let them.
"We will redouble our efforts to wipe out this poisonous, extremist ideology."
Scotland Yard will urgently review its tactics for responding to a marauding gun assault by terrorists in the wake of the Paris massacre.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said the scale of the attacks and the range of weaponry used in the French capital are a "serious cause for concern".
A London-educated lawyer was one the victims killed in the Bataclan massacre.
Valentin Ribet, a London School of Economics (LSE) business law graduate, has been confirmed as one of the 80 people feared dead after gunmen opened fire into the crowd at the concert hall in Paris.
Mr Ribet, 26, was a "talented lawyer" in the Paris office of international law firm Hogan Lovells.
The Queen sent her condolences to the French people in a message to the president, telling Mr Hollande that she and the Duke of Edinburgh were "deeply shocked and saddened by the terrible loss of life".
Mr Hollande denounced the attacks as an "act of war" and vowed that France would be "merciless" in its response to the "absolute barbarity".