It's the highest level seen since "Good Lord!" when the Argies piped up.

Britain raised its terror threat level to “severe” on Friday, meaning that officials consider an attack on the country is “highly likely” -- an assessment based on the growing influence of the Islamic State in Iraq and Syria.

At a news conference in London, British Prime Minister David Cameron said the Islamic State poses a “greater and deeper threat to our security than we have known before.”

However, though the country lifted its threat level one notch from “substantial” to “severe,” the second-highest category, Home Secretary Theresa May stressed that “there is no intelligence to suggest that an attack is imminent.”

It was the first time in three years that Britain has been at such a heightened security level.

The move comes as international attention has focused on the Islamic State, an Al Qaeda splinter group, after it beheaded American journalist James Foley. In an online video of the killing, the executioner speaks with a British accent.

Cameron said Britons were “shocked and sickened” by idea that Foley may have been killed by someone who is British.

“It was clear evidence ... that this is not some foreign conflict thousands of miles from home that we can hope to ignore,” the prime minister said. “The ambition to create an extremist caliphate in the heart of Iraq and Syria is a threat to our own security here in the U.K.”

May, similarly, said the increase in 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 traveled there from the U.K. and Europe to take part in the conflict.”

It’s estimated that approximately 500 Britons have traveled to fight in the region. About 200 are believed to have already returned home, and concern is growing in Britain about a potential influx of fighters flowing back into the country.

To combat that threat, Cameron announced plans for legislation that will allow Britain to strip suspected terrorists of their passports. He promised more details would be presented to Parliament on Monday.

“We need to do more to stop people traveling, to stop those who do go from returning, and to deal decisively with those who are already here,” the prime minister said.

Britain raises terrorism threat in response to Islamic State - LA Times