JAPAN: An unsuspecting traveller who walked away from a Tokyo airport with one million yen's worth of cannabis due to a customs blunder has returned the drugs, an official said on Friday (May 30).

The passenger, identified only as a foreign person, was "extremely cooperative" and handed the drugs back to authorities after being informed of the mistake, a customs spokeswoman said.

An officer at Narita International Airport on Sunday stuffed 142 grams of the drug into the side pocket of a randomly selected black suitcase coming off an overseas flight for a sniffer dog to find.

The animal failed the test and the 38-year-old customs officer forgot which bag it was put in.

"We tried to call passengers one by one and finally we found the person late Monday," the spokeswoman said.

The cannabis, which has a street value of one million yen, was in a metal box wrapped with newspapers.

The officer admitted to being aware of rules against using passenger bags for training but decided to do it anyway to "improve the sniffer dog's ability."

"We are relieved to have recovered the drug," the spokeswoman said, adding that the officer will be punished.

Japan prohibits cannabis and possession of even small amounts can lead to imprisonment.