The UK government has stepped up security at borders, including ports and at checkpoints on French soil, after deadly terror attacks in Paris.
Home Secretary Theresa May said the extra measures were not based on specific intelligence, but followed the "cowardly" assault on weekly satirical newspaper Charlie Hebdo and the murder of a French police woman.
"Following the attacks, we took the precautionary step yesterday of increasing security at the French/UK border," she said.
"The UK threat level - which is set by the independent Joint Terrorism Analysis Centre (JTAC) - remains at 'severe'. That means that a terrorist attack is 'highly likely' and the public should remain vigilant. And this morning I chaired Cobra to consider Britain's response to the attacks and our own preparedness for a similar attack."
May added: "People from all faiths and walks of life have expressed their disgust at the events that took place in Paris. I want to reiterate our commitment to standing with the French people against terror. The thoughts and prayers of all of us are with the families, friends and colleagues of the victims."
French authorities have confirmed that eight journalists and two policemen were among the 12 people killed.
Prime Minister David Cameron condemned the murders as a "sickening" attack on press freedom.
Labour leader Ed Miliband joined the Prime Minister in expressing his "horror and outrage" at the events in Paris.
Foreign Secretary Philip Hammond said he is "appalled" by the attack in Paris. He tweeted: "My thoughts are with the family and friends of those killed."
Paris police said one suspect, Hamyd Mourad, had handed himself in, while two other suspects, French nationals Said Kouachi and Cherif Kouachi, were at large.