The Iranian rial plummeted by as much as a quarter this week as fresh oil sanctions and a ban on unauthorised foreign exchange trading took their toll on the Iranian economy.

The chief driver of the collapse has been US-led oil sanctions imposed on the country's chief export, crude oil. Fresh economic sanctions from the EU are due to be imposed imminently. Since oil is priced globally in dollars, it provides a crucial source of dollar income for the country, allowing it to operate a functioning currency market in what is effectively a closed economy.