The huge trade in sending waste to China is fraught with major problems, according to the world's recycling trade association.
The Bureau of International Recycling (BIR) says many exporters are in "serious disarray" due to the Asian country's exporter registration system.
China recently published a list of approved waste exporters - and declared that all others who had applied by July 31 last year had been unsuccessful.
But BIR believes that far from clearing up uncertainties in the global trade, this latest list has added to the confusion.
A spokesman said: "The BIR has officially contacted the General Administration of Quality Supervision, Inspection and Quarantine of China (AQSIQ) to express its deep concern regarding a number of unresolved issues.
"In a letter sent to the head of AQSIQ's Department for Supervision and Inspection, BIR questioned the way that Chinese legislation was implemented regarding overseas suppliers' registrations."
A major problem BIR highlighted in the letter was that applicants were not told which materials their licence covered them for, leading to shipments being rejected at ports.
Secondly, the trade association said companies who had their application rejected were not told why so they are finding it hard to re-apply.