The environmental activist group said it would file a criminal complaint with the Tokyo District Public Prosecutor's Office the same day against 12 crew members, using a box of meat it obtained as evidence.

Greenpeace said a four-month investigation had found that crew members of the Nisshin Maru factory ship had smuggled the meat ashore disguised as personal luggage and passed it on to traders to be sold illegally.

The ship's operator, Kyodo Senpaku, said in response that there had been a decades-old custom of giving crew members 'souvenir' meat so that they could distribute it to neighbours when they returned home.

But the firm was 'worried' that some wrongdoing may have taken place, a company official said.

'There is suspicion', said the official who declined to be named.

Japan, which kills whales using a loophole in a 1986 whaling moratorium that allows 'lethal research' on the giant mammals, says that it is monitoring whale numbers, but makes no secret that the meat ends up on dinner tables.

Japan argues that the whale meat, a 'by-product' of the research, should be consumed rather than wasted. The meat is supposed to go to wholesalers at a price set by the government-backed Institute of Cetacean Research.

The whaling fleet returned home in April having caught little more than half of its original target of about 950 whales after a series of high-seas clashes with militant environmentalists.

Greenpeace said it had documented the offloading of smuggled whale meat into a special truck when the Nisshin Maru docked, working from information from former and current Kyodo Senpaku employees.

One of four boxes destined for a private address was then intercepted and found to contain 23.5 kilogrammes of salted whale meat worth up to US$3,000 (S$4,100), despite being labelled as cardboard, it said.

The group took the box as evidence.

One informer told Greenpeace that dozens of crew take as many as 20 boxes each, it said.

'This will damage Japan's credibility abroad and at home because this means taxpayers' money is being stolen. This is a very serious crime,' Mr Jun Hoshikawa, head of Greenpeace Japan, told a press conference. -- AFP