A Briton has been confirmed dead following the terrorist attack in Paris and officials fear that a "handful" of people from the UK may also have lost their lives in the atrocities.
A Foreign Office spokeswoman confirmed the death of a British national and said next of kin had been informed.
But a Government source indicated the British death toll was expected to rise.
"The picture is still unclear," the source said.
"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."
Islamic State (IS) has claimed responsibility for the attacks which killed at least 127 people and wounded 300 more, around 80 of them critically.
A Foreign and Commonwealth Office spokeswoman said: "It is with regret that we can confirm that a British national has been killed in the Paris terror attacks.
"Next of kin have been informed but have asked for privacy and time to come to terms with the news before further details are released."
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.
Mr Cameron chaired a meeting of the Government's emergency committee and also spoke to Mr Hollande following the atrocities.
Mr Cameron 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".