Significant territory losses means the number of people living in the Jihadi caliphate slumped from nine million at the start of 2015 to fewer than six million, according to the tax report by the US-based consultancy firm IHS.
It has caused the dreaded groups' tax receipts to plummet from around USD 80 million each month to USD 56 million, the report said.
Once branded as the "richest terrorist group" in the world, the ISIS is now in crisis as the territory under its control has declined by about 22 per cent since mid-2014.
"Our research has found that the Islamic State is increasing taxes on basic services and coming up with new ways to get money from the population," Carlino was quoted as saying by the 'Daily Express'.
Carlino also claimed that the terror group slapped taxes on "installing satellite dishes" and "exit fees" for people trying to leave a city.
The report also said that oil production in areas under the jihadist group's control had gone down to 21,000 barrels per day from 33,000 barrels per day.
ISIS receives half its revenue from taxation and confiscation while selling and smuggling oil makes up 43 per cent of its cash.
But both income streams have been badly affected after its territory shrank by nearly a quarter.
ISIS, which controls large swathes of territory in Iraq and Syria, were recently pushed by Iraqi forces towards the center of a town held by the dreaded outfit in western Anbar province.
The group was also driven out of Palmyra and Russian forces have now entered the city.
More than 270,000 people have been killed in the Syrian war since its eruption in 2011.
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