The two warships, with a combined crew of 420, will set sail next Monday to replace two destroyers that have been in the Gulf of Aden since March.

“I ordered to send two patrol ships on July 6 to the Gulf of Aden off Somalia so that they will start their missions at the end of July,” Defence Minister Yasukazu Hamada told reporters.

Because of limits on Japan's military imposed by the country's post-World War II pacifist constitution, its destroyers there now have no mandate to use force except to protect Japanese interests or when acting in self-defence.

But parliament passed an anti-piracy law in June that allows the Maritime Self-Defence Force to protect any commercial ships threatened by pirates, not just those sailing under the Japanese flag or carrying Japanese nationals or cargo.

The new law also widens the navy's rules of engagement and allows it to fire at the hulls of pirate vessels that approach other ships, as a last resort.

Hamada said that the new legislation takes effect on July 24, and that “the crew members have been trained to carry out the new mission.”

Japan in March joined the United States, China and more than 20 other countries in the maritime operation against pirates who have attacked ships in the waters off the Horn of Africa, a key route leading to the Suez Canal.

In addition to the destroyers, Japan last month also dispatched two maritime surveillance aircraft and scores of military personnel to the region to beef up its anti-piracy mission.