The al Qaida-inspired extremist group behind an ongoing and bloody insurgency in Iraq has been banned under UK terrorism laws.
The Islamic State of Iraq and the Levant, known as ISIS, is one of five terror organisations proscribed by the Home Office, making membership and support for them a criminal offence.
The move was triggered by the jihadist organisation's actions in Syria, but Home Office officials said developments in Iraq underlined the need to ban the group in the UK.
Militants led by ISIS have captured a number of key cities across Iraq, including Mosul and Tikrit, with commentators warning the whole country is on the brink of full-blown civil war.
Four other groups with links to Syria to be proscribed are Turkiye Halk Kurtulus Partisi-Cephesi, also known as the Turkish People's Liberation Party or The Hasty Ones, Kateeba al-Kawthar, Abdallah Azzam Brigades, including the Ziyad al-Jarrah Battalions and Popular Front for the Liberation of Palestine - General Command.
Immigration and security minister James Brokenshire said: " We condemn all acts of terrorism committed in Syria, where the conflict poses a growing threat to UK, regional and international peace and security.
"Proscription is a useful weapon in the armoury at the disposal of the Government, police and security service to disrupt terrorist activity and protect the UK.
"Today we have laid an order which will proscribe five groups with links to Syria. Four other groups operating in Syria are already proscribed. This means being a member of or supporting these organisations will be a criminal offence.
"We want to send a strong message that terrorist activity is not tolerated wherever it happens."
ISIS, led by Abu Bakr al-Baghdadi, ultimately wants to create an Islamic state across Sunni areas of Iraq and in Syria.
As Syrian president Bashar al-Assad lost control over large parts of the north of Syria, and along the border with Iraq, it seized the opportunity to grow, recruit foreign fighters and take control of parts of Syria
Foreign Secretary William Hague has insisted that Britain would not be responding militarily to the critical situation developing in Iraq, despite international outrage over pictures showing jihadist fighters apparently massacring captured government troops.
US president Barack Obama is weighing up what help to give Baghdad to counter ISIS, while t he Pentagon has sent an aircraft carrier to the Gulf in advance of potential air strikes.
Former prime minister Tony Blair yesterday argued in favour of a tough response to the extremist insurgency - arguing it was caused by a failure to deal with the Syria crisis, not the invasion of Iraq by US and British forces 11 years ago.
His remarks sparked a furious reaction from London Mayor Boris Johnson, who accused the ex-Labour leader of being ''unhinged'' and having sent UK forces into the bloody conflict in part to gain personal ''grandeur''.