Iran’s illicit gold trade has become a lifeline for the regime as it battles to stave off economic collapse under the weight of international sanctions.

Investigators trying to track Iran’s gold purchases say that the trade has rocketed since Tehran was expelled from the Swift electronic payment system last year. That punishment made it almost impossible for the Government to pay for imports in foreign currency.

Gold has become more important since the latest sanctions, which came into force last week, obliged countries still buying Iranian oil to do so only in their local currency through quarantined accounts.

The United States has made clear it would consider any breach of these sanctions as money-laundering. China, Iran’s biggest oil customer, is understood to have taken legal