The UK’s terror threat level is being raised from "substantial" to "severe" in response to conflicts in Iraq and Syria, Home Secretary Theresa May says.
The new alert level means an attack is "highly likely", although Mrs May said there was no intelligence to suggest an attack on the UK was "imminent".
It is the second highest of five possible UK threat levels.
It comes amid concern about hundreds of UK nationals who are thought to have travelled abroad to fight.
Efforts are also under way to identify a suspected British jihadist who appeared in Islamic State (IS) footage of the killing of US journalist James Foley.
National Policing Lead for Counter-Terrorism Assistant Commissioner Mark Rowley said security and protection measures were being increased from Friday.
‘Strongest possible action’
In a statement Mrs May said: "The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West.
"Some of those plots are likely to involve foreign fighters who have travelled there from the UK and Europe to take part in those conflicts.
"The first and most important duty of government is the protection of the British people.
"We have already taken steps to improve our powers and increase our capabilities for dealing with the developing terrorist threats we face. That process will continue and the British public should be in no doubt that we will take the strongest possible action to protect our national security."
She said the UK faced a "real and serious" threat from international terrorism, urging members of the public to remain vigilant.
UK terror threat levels
Critical – attack expected imminently
Severe – attack highly likely
Substantial – attack a strong possibility
Moderate – attack possible but not likely
Low – an attack unlikely
Source: Home Office
Mrs May said the decision to raise the threat level was made by the Joint Terrorism Analysis Centre (JTAC).
She added: "JTAC’s judgements about that threat level are made on the basis of the very latest intelligence and are independent of ministers.
"JTAC has today raised the threat level to the UK from international terrorism from substantial to severe.
"That means that a terrorist attack is highly likely, but there is no intelligence to suggest that an attack is imminent."
AC Rowley said: "From this afternoon we will begin to increase our levels of visible patrols and implement other security and protection measures.
"We will also build on existing community relations to provide reassurance and seek their support and assistance in keeping the UK safe."
He urged communities and families to report anyone who is "vulnerable, a danger or escalating towards terrorism" by calling the Anti-Terrorist Hotline on 0800 789 321.
The terror threat level has been made public since 2006 and has twice been raised to "critical" – in August 2006 after police uncovered a plot to blow up passenger planes, and in June 2007 after an attack at Glasgow Airport.
The level last changed in July 2011 when it was reduced to "substantial"
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