People living in territory controlled by the extremists are forced to pay a religious tax, to fund salaries that attract recruits.
An investigation found that the terror group now earns as much from taxation, extortion and confiscation as oil, which brought in a staggering £296million ($450m) during the past year.
Wealthy residents and businesses are forced to give 2.5 per cent of capital to ISIS tax collectors, while farmers must hand over the equivalent of five per cent of irrigated crops and 10 per cent of rain-fed crops.
As part of its investigation, the Financial Times spoke to Mansour, a 26-year-old grocery shopkeeper in eastern Syria, who delayed paying the tax when it was introduced last summer and subsequently received a visit from four ISIS officials.
They stormed his shop and calculated his tax bill based on the retail price of his stock, billing him 32,500 Syrian lira, equivalent to about £71.
Speaking via an internet site, Mansour said: 'They told me, 'You liar . . . How will victory be achieved if you're not paying tax?'daily mail reported.
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