The killings were “absolutely horrific”, but the terror threat would be beaten. “This is, I think, going to be the struggle of our generation and we have to fight it with everything we can,” Cameron told BBC Radio 4’s Today programme.
A total of 38 people were killed on a beach near Sousse by a gunman with links to IS extremists.
Cameron said IS could be beaten but it would take “a very long time”.
He said IS posed “an existential threat” to the West, and its members in Iraq and Syria were plotting “terrible attacks” on British soil.
Asked if British troops were needed on the ground, he added: “Our strategy is to build up local armies. It’s much easier to just invade a country… it’s easier and faster, but that has consequences.”
Cameron, who will chair another meeting of the Cobra emergency committee on Monday morning, also said an RAF C17 transport plane would be sent to Tunisia on Monday to help evacuate casualties, and if families wished, aircraft could help return their loved ones’ bodies home.
A number of British tourists remain missing, with their relatives continuing to face an agonising wait for news.