The Syrian government cut deals with Islamic State to help the jihadists earn more than $40 million a month from the sale of oil, documents recovered from a US and British raid on a key commander have revealed.

Thousands of spreadsheets and accounts kept by the group’s oil boss Abu Sayyaf, retrieved in the biggest intelligence raid in US Special Forces' history last year, reveal how the two sides forged a mutually beneficial arrangement despite being at war with one another.

Isil fighters captured some of the state’s best-producing oil fields of eastern Syria in 2013.

The Telegraph reported claims two years ago that the regime had been purchasing oil from the jihadists, but the documents, seen by the Wall Street Journal, show the scale of the collusion.

At the height of production in late 2014 to early 2015, Isil recorded $40.7m profit each month - the lion’s share of which made from sales to the Syrian government, according to the US Treasury Department.