The British public need to be prepared for the fact that many of those killed when a gunman massacred holidaymakers on a Tunisian beach are British, the Prime Minister has said.
He spoke as the UK death toll rose to eight out of 39 people killed in the popular resort of Sousse.
Speaking in Downing Street, Cameron said he could only imagine the "pain, and anguish and agony" of families caught up in these "terrible events", adding: "We'll do whatever it takes to help them."
"These were innocent holidaymakers relaxing and enjoying time with their friends and families. Like the victims in France and Kuwait yesterday, they did not pose a threat to anybody.
"These terrorists murdered them because the terrorists oppose people and countries who stand for peace, tolerance and democracy.
"Wherever they are in the world, these terrorists will not succeed, for as much as they try to divide people they will only unite us more strongly in our determination to defeat these Islamist extremists and all they stand for."
Asked about the increased security in place today for events across the UK marking Armed Forces Day, Cameron went on: "Let's be clear. There's no doubt we face a very severe threat in our country and we have done for many months and many years, but the level of the threat is identified independently of government and published in the proper way.
"But today is about paying tribute to our armed forces and their families who do so much to keep us safe and we should proceed with those events."
But he said if people worked together, were vigilant and backed the police, "we can combat this poisonous narrative of Islamist extremism that is radicalising too many young minds in our country".