ISIS claim to have hacked the personal information of hundreds of military, political and diplomatic personnel and released it online in a reported 'hitlist'.
The list, in the form of a spreadsheet and shared by the so-called 'Islamic State Hacking Division', contains the private details of 1,400 individuals.
In a shocking warning to those named, an accompanying message reads: 'Know that we are in your emails and computer systems, watching and recording your every move, we have your names and addresses, we are in your emails and social media accounts.'
'We are extracting confidential data and passing on your personal information to the soldiers of the khilafah, who soon with the permission of Allah will strike at your necks in your own lands!'
The list is said to include British Foreign Office workers and even a local council employee, Sky News reports.
The extensive spreadsheet is understood to also include the names and contact information of hundreds of United States Department of State figures and Australia officials.
The job title, computer passwords, email and phone numbers are all understood to be included in the list. The document was removed by officials on Wednesday night.
British jihadist Junaid Hussain who is now fighting with Islamic State in Syria, shared the list on his Twitter page.
Hussain, believed to be living in the ISIS stronghold of Raqqa, is the former leader of a shadowy computer hacking group known as Team Poison.
He was jailed in 2012 for stealing personal information from Tony Blair and publishing it online. He also admitted making hoax calls to the Metropolitan Police anti-terrorist hotline.
Hussain and his wife Sally Jones, who together have been dubbed 'Mr and Mrs Terror', are thought to have fled Britain to join ISIS in 2013.
He was named earlier this week in an investigation into Syria-based jihadis reported to be inciting attacks in Britain.
Australian Neil Prakesh, who is also known as Abu Khaled al-Cambodi, also shared the link.
'Kill them where you find them and enslave their women,' Prakesh wrote on his social media at 4.30am on Wednesday morning, according to Australia's Herald Sun.
'They have us on their 'hit list,' and we have them on ours too...' he wrote.
The Islamic State Hacking Division social media account was suspended three times, before a fourth page was created, bragging they were 'back again.'
A Foreign Office spokesman said the department does not comment on security issues.
By Lucy Mae Beers, Richard Spillet
© Associated Newspapers Ltd.