Theresa May has called a meeting of the Government's emergency Cobra committee, while the London Mayor has said security in the capital will be reviewed after the Bastille Day attack in Nice.
The Prime Minister said Britain stands "shoulder to shoulder" with France after the "horrifying" terror attack on the French Riviera that left at least 84 people dead.
Mrs May, who became Prime Minister on Wednesday, said she had called a meeting of senior officials in the Government's Cobra committee to review security and see what Britain could do to help.
Sadiq Khan, the London Mayor, said he would be "reviewing our own safety measures'' in light of the attack in Nice.
Sources said the review was likely to look at policing and security arrangements for major public events in the capital.
Speaking ahead of a visit to Scotland, the Prime Minister said: "I have asked my deputy national security adviser to chair a Cobra meeting of senior officials to review what we know and what we can do to help and I will speak to President Hollande today and make clear that the United Kingdom stands shoulder to shoulder with France today as we have done so often in the past.
"If, as we fear, this was a terrorist attack then we must redouble our efforts to defeat these brutal murderers who want to destroy our way of life.
"We must work with France and our partners around the world to stand up for our values and for our freedom."
Mrs May said the Government was working to establish whether any British nationals were caught up in the attack.
"Our hearts go out to the French people and all those who have lost loved ones or been injured," she said.
"While the full picture is still emerging, it seems that at least 80 people are feared dead and many others have been injured.
"It was innocent victims enjoying a national celebration with their friends and families.
"We are working urgently to establish whether any British nationals were caught up in the attack.
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"Our ambassador is travelling to Nice today with consular staff and they will be doing all they can to help anyone affected."
She added: "We are working and (are) aware of reports that there has been a Briton involved but we are working urgently to establish if there are British nationals involved who have been caught up in this attack."
Mrs May said officials at the Cobra meeting would look at whether security needs to be stepped up in the UK in the wake of the attacks.
"The threat level here in the United Kingdom is already at severe - that means that a terrorist attack is highly likely," the PM said.
"Senior officials today will be reviewing what more we can do to ascertain whether there is any further action we need to take.
"But our security services and our police are ever-vigilant and, as I say, the threat level here is already at a level which says that a terrorist attack is highly likely."
Mrs May continued: "The Government is already making more funding available for the security services which will enable them to increase their efforts.
"It's also making more funding available to the police for counter-terrorism policing.
"Of course we always look at these issues - we have been living with a severe threat level for some time now - but today our thoughts must be with those people in France, with the people of France, and all those who have lost their loved ones or have been injured in this attack."
