Britain's government has raised the country's terror threat level to "severe," Home Secretary Theresa May said on Friday.
It's the second highest of five potential threat levels. According to the U.K. government, it means an attack is "highly likely." An attack may be imminent.
May said the threat level was raised because of the risk from the deteriorating situation in Iraq and Syria, where extremist militants from the group calling itself the Islamic State (also ISIS or ISIL).
"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 these plots are likely to involve foreign fighters who have traveled there from the UK and Europe to take part in those conflicts," May said in a statement.
"The first and most important duty of government is the protection of the British people," May said. "We have already taken steps to amend 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."
In a statement delivered from Downing Street in London Friday afternoon, U.K. Prime Minister David Cameron said Britain was facing a "greater and deeper threat to our security than we have known before." He said confronting ISIS was part of a generational struggle that he thought could last "decades."
"Poisonous ideology of Islamist extremism is the root cause of the terror threat," Cameron said. "We will always act with urgency when needed."
The West — particularly the U.S. and U.K. — have increased their warnings about ISIS in the wake of the group's brutal murder of American journalist James Foley last week. A 23-year-old former British rapper is considered a prime suspect in Foley's beheading. Cameron said Friday that it "increasingly seems to be a British terrorist recorded on that video" of Foley's murder.
At least 500 people from Britain have traveled to Middle Eastern regions to join ISIS, Cameron said. Western officials have warned about the possibility of ISIS fighters holding Western countries' passports traveling back to those countries to plan and possibly launch attacks.
Cameron said there was "no doubt" in his mind that ISIS is "targeting all of us in Western Europe." He did not commit to joining the U.S. and possibly other allies in taking military action against the group, saying it was just "one element of what we can do."
"We must use aid, diplomacy, and our military," he said.
"We are in the middle of a generational struggle against a poisonous extremist ideology that I believe we will be fighting for years and probably decades."
Warning that "ISIS represents a greater threat than anything before," he plans to introduce new laws to "make it easier to take people's passports away."