In a rare public appearance, MI-5 Director General Jonathan Evans told the BBC his agency is working hard to counter what he termed an "astonishing" level of attacks.
"This is a threat to the integrity, confidentiality and availability of government information but also to business and to academic institutions," Evans said.
He made the comments during a speech Monday night at London's Mansion House.
Evans said this summer's 2012 Olympic Games are an attractive target for terrorist groups but added that security preparations are well under way.
He warned that the threat of terrorism did not evaporate with the death of Osama bin Laden.
"We seem to be moving from a period of deep and focused threat to one where the threat is less monolithic but wider," he said. "Today parts of the Arab world have once more become a permissive environment for al-Qaida."