Britain raised the terror threat level from substantial to severe Friday, meaning that a terrorist attack is considered highly likely.
Home Secretary Theresa Mays said the decision to raise the threat level was related to developments in Iraq and Syria, but that there was no information to suggest an attack was imminent. Some of the plots are likely to involve fighters who have traveled from Britain and Europe to take part in fighting in the Middle East.
"We face a real and serious threat in the UK from international terrorism," she said. "I would urge the public to remain vigilant and to report any suspicious activity to the police."
May says the decision by the Joint Terrorism Analysis Center is made on the basis of intelligence and is independent of government. "Severe" is the second-highest of five levels.
British police have appealed to the public to help identify aspiring terrorists after the murder of an American journalist focused attention on extremism in the UK.
The involvement of a person of British nationality in James Foley's murder underscored the need to identify those who might travel abroad to fight or are at risk of being radicalized.