"If verified, this is a despicable and barbaric murder," Cameron said in a statement yesterday.
"As I have said consistently over the last few weeks, ISIL terrorists speak for no religion. They threaten Syrians, Iraqis, Americans and British people alike and make no distinction between Muslims, Christians or any other faith."
Cameron expressed his sympathy with the family and friends of reporter Steven Sotloff, the second US journalist to be killed by the group in recent weeks.
The prime minister said he would hold a Cobra emergency meeting today to "review these latest developments" and said the government would do all it could to protect its people from the jihadists.