Drugs are the means of circumventing capital flight limits.

The underground banks operate largely by selling the cartels’ dollars to wealthy Chinese who, alarmed at the political tightening under Chinese leader Xi Jinping, are looking to circumvent capital controls and transfer their money out of the country.

In essence, the officials claim, a new global money laundering network has evolved that marries two powerful financial forces — the huge stockpiles of cash being accumulated by the Mexican drugs cartels from selling drugs in the US, and the rapidly growing volumes of capital seeking an escape route from China.

“Chinese money launderers have taken over laundering for the Mexican drug cartels,” says Chris Urben, who worked at the DEA for 24 years, investigating traffickers around the world. This alliance, he adds, is “amazing to watch . . . a game-changer”.

In interviews with the Financial Times over the past few months, more than a dozen current and former DEA agents and high-ranking law enforcement officials in the US, UK and Italy have raised the alarm about a method of money laundering that they say is evading standard forms of surveillance and is thwarting their efforts to pursue drug traffickers.