Express mail service has become an accomplice of drug smuggling, according to Shanghai customs.

Eight of the nine drug smuggling cases intercepted at Shanghai ports this year came via parcel posts, while the number was 16 out of the 28 cases last year, data from customs show.

People who sign to receiving the mail in these cases are usually not the drug dealers, said the authority, and young women are mostly used by lawbreakers as smuggling tools.

In one typical case, a man asked a woman if she could receive a parcel of clothes from overseas as a favor. It turned the parcel had more than 700 grams of heroin inside.

Customs staff reminded the public not to collect mail for others.

Jiang Wenqing contributed to this story.