Speaking outside a global conference on internet security in London, Eugene Kaspersky, a Russian maths genius, told Sky News the threat was a real and present danger.
"I don't want to speak about it. I don't even want to think about it," he said. "But we are close, very close, to cyber terrorism. Perhaps already the criminals have sold their skills to the terrorists -- and then ... oh, God."
Kaspersky, who founded an internet security empire with a global reach, said he believed that cyber terrorism was the biggest immediate threat confronting nations as diverse as China and the US.
"There is already cyber espionage, cyber crime and hacktivisim [when activists attack networks for political ends] -- soon we will be facing cyber terrorism," he said.
UK Prime Minister David Cameron, talking at the London Cyber Conference, added to the growing chorus of world leaders sounding the cyber alarm.
"We are here because international cyber security is a real and pressing concern," he said. "Let us be frank. Every day we see attempts on an industrial scale to steal government secrets -- information of interest to nation states, not just commercial organizations.
"Highly sophisticated techniques are being employed ... These are attacks on our national interest. They are unacceptable."
He warned that "we will respond to them as robustly as we do any other national security threat."
Both the US and UK used the conference to set out principles they hope will form the basis of international cooperation in web governance, in which states would work together on issues such as security and copyright protection without imposing new restrictions on users, The Wall Street Journal reported.
The conference, which was attended by business and government leaders from around the world, demonstrates how cyber security has vaulted on the foreign policy agenda. But it is as likely to highlight disagreements as much as consensus, with China and others as interested in clamping down on internet users than shutting the door on criminals and spies.
"How do we achieve security for nations, people and business online without compromising the openness that is one of the internet's greatest attributes?" US Vice President Joe Biden said to the conference via video link.
Secretary of State Hillary Clinton had canceled her attendance due to the death of her mother.
UK Foreign Secretary William Hague said whatever disagreements emerge, the speedy development of the internet means discussions of its future and governance need to move on to an international stage.
"The truth is that in cyber space, no one country can go it alone," he told the conference in his opening address Tuesday. "In the place of today's cyber free-for-all, we need rules of the road."
Hague announced seven principles as the basis for more effective cooperation, including "the need for governments to act proportionately" in cyberspace and in accordance with international law; protection of freedom of expression; respect for privacy and copyright; and proposed joint action against criminals acting online.
A US official said the principles were largely in line with US cyber strategy and the conference was significant because it helped move the internet from just a technical discussion to one of international diplomacy.
Officials from around 60 nations are attending the two-day conference. Among those is China, which some officials in the UK and US have accused of orchestrating a campaign of cyber espionage aimed at stealing the intellectual property of their largest companies.
