An unwitting passenger arriving at Japan's Narita airport has received 142g of cannabis after a customs test went awry, officials say.  

A customs officer hid a package of the banned substance in a side pocket of a randomly chosen suitcase in order to test airport security.  

Sniffer dogs failed to detect the cannabis and the officer could not remember which bag he had put it in.  

Anyone finding the package has been asked to contact customs officials.  

"This case was extremely regrettable. I would like to deeply apologise," said Narita International Airport's customs head Manpei Tanaka.  

The customs officer conducted the test on a passenger's bag against regulations. Normally a training suitcase is used.  

"I knew that using passengers' bags is prohibited, but I did it because I wanted to improve the sniffer dog's ability," the officer was quoted as saying.  

"The dogs have always been able to find it before... I became overconfident that it would work," he said.  

Japan has strict laws against drugs and possession of small amounts of cannabis can lead to a prison sentence.