A number of Britons are feared dead following the terror atrocities in Paris, David Cameron has said.
The Prime Minister condemned the “brutal and callous murderers” behind the series of attacks in the French capital.
“We must be prepared for a number of British casualties,” he warned.
Speaking in Downing Street he said the terror threat level in the UK would remain at “severe”, but the attack would prompt a review of plans and suggested the threat posed by Islamic State was “evolving”.
In a message of solidarity to the people of France he said: “Your values are our values, your pain is our pain, your fight is our fight.”
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.”
Mr Cameron said the full picture of the attacks was still emerging but “we must be prepared for a number of British casualties and we are doing all we can to help those caught up in the attack”.
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.”