Stuart Osborne, senior national coordinator for counter-terrorism, said the threat of a terrorist attack "remains a strong possibility and could occur without warning," The Daily Telegraph reported Thursday.
Osborne, who is also deputy assistant commissioner of the Metropolitan Police, said threats come from al-Qaida-inspired Islamic groups in various parts of the world.
Other threats against Briton, he said, come from groups in Northern Ireland.
Osborne said an increasing number of Britons were using conflicts abroad, particularly in Syria, to become trained in terror strategy before returning to the United Kingdom and forming small groups that are hard to detect.
"We are seeing more small groups getting together at shorter notice and more people wanting to do things without that broader command and control [of al-Qaida]," he said.
The number of terrorism-related arrests rose 60 percent in the year that ended in September 2012, the Home Office recently revealed. About 245 people were held on suspicion of terrorism-related offenses during that period.