Cameron, who was addressing a World Economic Forum event here, highlighted on the negative impact of Brexit, which would be welcomed only by Russian President Vladimir Putin and Abu Bakr al-Baghdadi, the chief of ISIS.
"Who would be happy if we left? Putin might be happy. I suspect al-Bagdadi might be happy," he said.
His latest comments, alluding to making the UK more vulnerable to terrorists in the event of an EU exit, will be described as scaremongering by the Leave camp, which says Brexit would give the UK greater control over its borders.
Cameron also vowed that the referendum will be a "once in a lifetime" event, rubbishing statements by some Leave campaigners raising the prospect of another referendum in the future.
"I'm absolutely clear that the referendum is the referendum. It is a once in a generation, once in a lifetime opportunity. The result determines the outcome - if we vote to stay, we stay, and that's it. If we vote to leave, we leave, and that's it. We can't have 'never-endums'," he said.
"In a 52-48 referendum this would be unfinished business by a long way. If the Remain campaign win two-thirds to one-third that ends it," he said.
The opinion polls are running neck-and-neck currently, with both camps at a nearly 50-50 vote.
The campaigning by both side will intensify in the coming weeks in an attempt to swing any undecided votes.