The United Nations is expected to place sanctions on four British jihadists accused of fighting or recruiting for the Islamic State group in Syria, the British government says.
The two women and two men would face a travel ban and an asset freeze to try to discourage others leaving Britain to join the jihadist group, which has seized control of large areas of Syria and Iraq.
"We will do all we can to stop British citizens from going to fight for ISIL," a spokesman for Prime Minister David Cameron said.
"These sanctions are a powerful tool – freezing an individual's assets and imposing a global travel ban on them ... sends a clear deterrent message."
At least 700 Britons are thought to have travelled to join extremists in the region where IS has been extending its influence amid the chaos caused by the devastating civil war in Syria that has killed 240,000 and displaced millions.
The British government said it was the first time in almost a decade it had asked the UN to add Britons to its Al-Qaeda sanctions list of jihadists, adding that it could ask for more people to be included.
The women were named as 21-year-old Asqa Mahmood, a former student suspected of leaving Britain to operate in a religious enforcement unit in the IS stronghold of Raqqa in Syria.
The other is Sally Anne Jones, 46, a former rock musician dubbed the "punk jihadi" by the British tabloids, who is accused of recruiting women online after travelling to Syria with her husband.
Also named were former supermarket security guard Abu-Said al-Britani, or Omar Hussain, 28 – who told the BBC in 2014 he would only return to Britain to "plant a bomb" – and former medical student Nasser Muthana, 21, who has appeared in IS propaganda.