Britain has upgraded its terrorist threat level to severe, indicating that an attack is likely, in a statement on Friday citing developments in the Islamic State conflict.

The Joint Terrorism Analysis Centre in the U.K. and Home Secretary Theresa May said that the threat level has been increased from substantial to severe, indicating an attack is highly likely but not imminent.

"The official threat level informs security professionals across the public and private sectors as they make decisions about the appropriate level of protection in place across the U.K.," May said in a statement. "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 U.K. and Europe to take part in those conflicts."

A large number of IS militants have been identified as from the U.K., the Prime Minister David Cameron has previously said.

"What we're facing in Iraq now with ISIL (Islamic State) is a deeper and greater threat to our security than we have known before," Cameron said at a news conference Friday, USA Today reported.

"With ISIL, we are facing a terrorist organization not being hosted in a country but actually seeking to establish and violently expand its own terrorist state. This is not some foreign conflict thousands of miles from home that we can hope to ignore. (It is a) threat to our own security in the U.K."

He also promised stricter laws to fight the issue, including seizure of passports. While identifying the conflict, Cameron highlighted the fact that it involved the borders of a NATO member, according to the The Associated Press.

And, he added, while al-Qaida had been an organization of terrorists, IS (formerly ISIL and ISIS) is an actual state run by terrorists.

"The first and most important duty of government is the protection of the British people. 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," May said. "We face a real and serious threat in the U.K. from international terrorism. I would urge the public to remain vigilant and to report any suspicious activity to the police."