By Jonathan Head
BBC News, Tokyo
Japan is to start imposing new regulations on Tuesday, drastically reducing its trade with North Korea.
Japan is still fuming over a kidnapping row with N Korea
The rules require all ships over 100 tons to have special pollution insurance - which few North Korean vessels have.
The measure is being viewed as a sanction by Japan against North Korea for failing to explain the fate of Japanese people kidnapped decades ago.
Japan is North Korea's biggest trading partner, after China and South Korea.
Officially, Japan is holding off on its threat to impose sanctions on North Korea.
The government wants to avoid provoking its unpredictable neighbour at a time when it is trying to coax it back into negotiations over its nuclear weapons programme.
But the impact of the new regulations here will be the same.
Last year more than 100 North Korean ships called at Japanese ports, unloading cargoes of sea food and cheap clothing, and taking back second-hand cars and luxury goods for the communist state's rulers.
But only 16 of those ships have the insurance they now need to visit Japan.
North Korea's exports to Japan were valued at around $200m (£104m) last year and are one of its few legitimate sources of foreign exchange.
It has been accused of earning significant amounts from smuggling illegal drugs - claims Pyongyang denies.
But Japan is gradually tightening the economic screws on North Korea.
It has already shut down remittances of cash to the communist state from ethnic Koreans living in Japan.
One local member of parliament here has even threatened to block the North Korean leader, Kim Jong-Il, from obtaining his favourite Japanese melons.