Britain has stepped up security at ports serving France following the attack in Paris, officials said today, as the MI5 intelligence agency offered French counterparts its "full support".
The terror threat level remains at "severe" and officials said the measures were a precautionary measure following the slaying of 12 people at the French satirical magazine Charlie Hebdo yesterday.
A huge manhunt for two brothers suspected of the massacre zeroed in on a northern French town today after the discovery of one of the getaway cars.
In rare public comments, MI5 Director General Andrew Parker said the attack was a "terrible reminder" of the threat facing Western nations.
He confirmed the national spy agency was offering its French colleagues their "full support as they respond".
"It is too early for us to come to judgements about the precise details or origin of the attack but it is a terrible reminder of the intentions of those who wish us harm," Parker said.
Home Secretary Theresa May, the interior minister, held a meeting of the COBRA crisis committee to consider Britain's response to the attacks and its own preparedness for a similar assault.
She said the government had offered France "every assistance necessary".
Searches have been stepped up on cars and trucks crossing the English Channel to British ports, while armed patrols have increased at the Eurostar terminal at London's St Pancras station.
There was also a more visible security operation at British border controls in France, such as at the Calais sea port and the Gare du Nord railway terminal in Paris.
The Joint Terrorism Analysis Centre (JTAC) raised Britain's threat level to "severe" in August -- the second-highest of five levels -- meaning that an attack is considered "highly likely".
"If there is an attack in a neighbouring country that has port crossings into ours, you would expect our authorities to be looking at doing all they can to keep us safe," a JTAC spokeswoman said.
"We will be... Doing all we can to stop any perpetrators or those linked with the attacks reaching the UK", she said, though the new measures are "not... Based on anything specific that suggests that is likely".
The British flag was flown at half-mast over Prime Minister David Cameron's Downing Street office on Thursday.
Cameron said: "We stand united with the French people following the horrific killings in Paris.
