Services that allow people to communicate without providing access to their messages pose a serious challenge to law enforcement efforts to combat terrorism and other crimes, Cameron said Monday.
He didn't name specific apps, but suggested those with encryption would not jive with new surveillance legislation he's looking to enact if he gets reelected this year. Such apps include WhatsApp, iMessage, Google Hangouts, Microsoft's Skype, CryptoCat, and more.
"In our country, do we want to allow a means of communication between people which, even in extremists ... that we cannot read" Cameron said, adding later, "No, we must not."
"The first duty of any government is to keep our country and our people safe," he said.
He didn't say how the government might enforce the legislation or keep people from downloading such apps.
His comments follow the wave of shootings in Paris last week by Islamic extremists. Being able to gather information about people's communications, be that communications records or actual content, could help authorities to thwart and investigate attacks, Cameron said.
But his comments also come at a time of increased concern over government surveillance, and the loss of digital privacy in general. On the same day Cameron delivered his remarks, in the U.S. President Obama announced plans for new legislation that would give Americans more control over their data online. A Consumer Privacy Bill of Rights, Obama proposed, would allow consumers to decide what pieces of their personal data are collected by companies and decide how the data is used.