The UK has raised the country's terror threat level from substantial to severe, its second highest level, as Prime Minister David Cameron announced plans to introduce new laws restricting travel for British citizens to Syria and Iraq.
The home secretary, Theresa May, said on Friday that the decision to raise the threat level was related to developments in Iraq and Syria, but that there was no intelligence information to suggest an attack was imminent.
Some of the plots are likely to involve fighters who have travelled from the UK and Europe to take part in fighting in the Middle East, she said.
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"We face a real and serious threat in the UK from international terrorism [... ] I would urge the public to remain vigilant and to report any suspicious activity to the police."
May said the decision by the Joint Terrorism Analysis Centre was made on the basis of intelligence and is independent of government.
Cameron said on Friday: "I will be making a statement in the House of Commons on Monday. This will include further steps to stop people travelling, with new legislation that will make it easier to take people's passports away."
"Severe" is the second-highest of five levels.
British police have appealed to the public to help identify aspiring terrorists after the killing of US journalist James Foley focused attention on extremism in the UK.
The involvement of a person of British nationality in Foley's beheading underscored the need to identify those who might travel abroad to fight or are at risk of being influenced by controversial ideologies.
Authorities say around 70 arrests have been made in the first half of the year for a variety of offences, including fundraising, preparing for aggresive acts and travelling abroad for training as a fighter.
The police say such arrests are being made at a rate five times greater than 2013.
The Islamic State group, formerly known as ISIL, grew out of the US-led war in Iraq, and entered the civil war in Syria last year. It attempted to take command of Nusra and denounced al-Qaeda's overall leader, Ayman al-Zawahiri, as "straying from the path".
It has recently taken over vast areas of neighbouring Iraq and its leader, Abu Bakr al-Baghdadi, earlier this year declared himself the caliph of territory captured on both sides of the border.
Reports suggest that a number of the fighters in the group hail from Western countries, with a majority coming from Britain. However, Al Jazeera cannot independently verify these reports.