"A 'perfect storm' of downgrades, lackluster macroeconomic data and the ongoing eurozone debt crisis transformed a retreat by investors into something approaching a stampede during May, evoking memories of 2008," Barclays said in a research report.

Barclays said May ended with total assets under management of $394 billion in commodities, down $35 billion from the previous month.

"The speed and extent of recent commodity price falls suggest market participants are anticipating a severe deterioration in the macroeconomic outlook," the bank said.

SECTOR OUTFLOWS

Energy saw the largest outflows in May, at $2.7 billion, the highest since December last year. Agriculture saw the second-biggest outflow at $2.3 billion, the highest since September last year.

For precious metals, outflows of $1.9 billion were the highest in exactly a year, the bank said.

"Commodity data flow is still healthy, and if the green shoots of recovery in China take root, a mild recovery in prices is likely. However, longer-term gains will likely remain capped by European leaders failing to enact a lasting solution to the crisis.