British extremists are among the "most vicious and vociferous fighters" in the Islamic State, a jihadism expert said.
Britons are now taking part in conflicts "in every way" including acting as suicide bombers and executioners, Shiraz Maher, from King's College London's International Centre for the Study of Radicalisation, said.
"Unfortunately the British participation in the conflicts now raging in both Syria and Iraq has been has been one of full participation, one that has seen them at the front lines, taking part in the conflict in every way," he told BBC Radio 4's Today programme.
"So we have seen British fighters out there operating as suicide bombers, we have seen them operating as executioners.
"Unfortunately they are amongst some of the most vicious and vociferous fighters who are out there. That is unfortunately just a part of their radicalisation."
A British family have told ITV News of their devastation after discovering a close relative is fighting with the Islamic State in Iraq.
A French journalist who was held hostage with James Foley in Syria has told ITV News it was "devastating" to see his murder.