Prime Minister David Cameron convened a meeting of his government's emergency response committee to discuss the attacks in Paris which left over 120 people dead, as UK police prepared to step up security.
Britain's terrorism threat level stands at "severe", the second highest category which means a militant attack is considered highly likely.
Cameron will chair the meeting of the committee, known as Cobra, later on Saturday in London, where at the French embassy, the flag had been lowered to fly at half-mast.
The British Prime Minister called the attacks "horrifying and sickening" on his Twitter feed, having said on Friday he would help France in whatever way he could.
London Metropolitan Police Service's assistant commissioner Mark Rowley told the BBC that Britain's threat level was under constant review, adding that policing across the country would be strengthened, with additional officers and checks.
"We're strengthening our policing stance at the moment across the country," he said on Saturday.
A spokesman for the Met said that additional police patrols across London's popular West End theatre and restaurant district were planned for Saturday evening.
Britain's "severe" threat level has been in place since August 2014, when it was raised largely due to the danger the authorities say is posed by Islamic State fighters and the hundreds of Britons who have joined them.
The highest level is "critical", which means an attack is expected imminently.
