The sum haemorrhaging from one of the poorest countries in the world has led officials to believe the money is from plundered Western aid projects and security or reconstruction contracts, it has been reported.
Proceeds from the country's rampant opium and heroin businesses also account for part of the sum, which is more than the Afghan government's entire tax revenue.
The money is packed in suitcases or even stacked on pallets and flown mainly to Dubai.
Customs records for legally declared money leaving the airport showed £2.1 billion left between since the start of 2007 and the end of February 2010.
The declared cash is likely to be a fraction of what actually leaves the country. Customs records are incomplete and money is also smuggled out unregistered.
Afghanistan's endemic corruption and Hamid Karzai's failure to tackle the problem have angered his western backers who are pouring in money to defeat the Taliban insurgency.