“In mid-2015, the Islamic State’s overall monthly revenue was around $80 million (71 million euros),” said Ludovico Carlino, senior analyst at IHS, which issues regular reports on IS-controlled territory.

“As of March 2016, the Islamic State’s monthly revenue dropped to $56 million,” Carlino said.

An IHS report also said oil production in areas controlled by IS jihadists had gone down to 21,000 barrels per day from 33,000 barrels per day before.

This was due largely to air strikes by the US-led coalition and Russia, although IHS warned the decline was only an “interruption of production” as jihadists were able to repair infrastructure quickly.

The report said around 50 percent of IS revenues come from taxation and confiscation of businesses and property, with 43 percent coming from oil.