The divine origins of Japan's imperial family come under scrutiny as it allows limited access to two burial sites.

Japan's imperial household agency is to open the doors to some of the country's mysterious imperial tombs early next year after decades of pressure from archaeologists, in a move expected to anger ultra-conservatives.

Experts have long been denied access to the hundreds of imperial mausoleums and tombs, which the agency regards as not so much cultural relics as sacred religious sites.

Some historians, however, put the agency's reticence down to fears that close inspection of the burial mounds could unearth evidence that shatters commonly accepted theories about the origins of the Japanese imperial family.

Members of archaeological and historical societies will be granted limited access to two tombs in February and March, the Kyodo news agency said, quoting imperial household sources. Excavation work will be prohibited and researchers will be permitted to enter only the tombs' fringes.

The mausoleums are those of the Meiji emperor (1852-1912) and Empress Jingu (170-269), wife of the Emperor Chuai, whose date of birth is unknown.

While the move by the agency - the opaque bureaucracy that runs the affairs of the imperial family - is unlikely to shed new light on the origins of what some believe is the world's oldest monarchy, for Japan's increasingly vocal ultra-right, even this modest concession is a step too far.

They subscribe to the ancient myth that holds that Japan's emperors are the direct descendants of the sun goddess Amaterasu Omikami, and that the current monarch is the latest in an unbroken line of 125 emperors stretching back more than 2,600 years to Jimmu in the seventh century BC.

Although the wartime emperor, Hirohito, renounced his divine status after Japan's defeat in 1945, ultra-nationalists regard his son, the current emperor, Akihito, as a living god, and have issued death threats to archaeologists involved in previous attempts to gain access to the tombs.

Their greatest fear is that proper inspections of the tombs will reveal compelling evidence that the Japanese imperial family originated from China and the Korean peninsula.

Akihito alluded to his Korean ancestry on his 68th birthday in 2001. In remarks that were ignored or played down by most of the domestic media, he said: "I for my part, feel a certain kinship with Korea, given the fact that it is recorded in the Chronicles of Japan that the mother of Emperor Kammu was of the line of King Muryong of Paekche."

Kammu reigned from 781 to 806 AD while Muryong ruled the Paekche kingdom in Korea from 501 to 523 AD.

Earlier this month, the agency reprimanded a member of staff for removing a contentious entry about the imperial family's background on Wikipedia's Japanese-language site.

The offending paragraph read: "There is a view that the imperial household agency may be afraid that historical facts may be discovered that could shake the foundations of the imperial system."

The employee received a "severe" verbal warning not for his editing, but for making the change on an agency computer, which can be traced by the operators of the online encyclopaedia.

The secrecy surrounding the sites means that no one can be certain what lies inside them. They may be the final resting places of the emperors themselves, accompanied by artefacts and treasures, or they may, as some believe, turn out to be nothing more than hollow mounds of earth.

There are an estimated 20,000 ancient burial mounds dotted around Japan, but the most important are the 896 imperial tombs, including those of 124 emperors, from Jimmu to Hirohito, who died in 1989.

Every year, envoys conduct Shinto rituals at the tombs and offer gifts from the emperor, a practice critics say violates the constitutional separation of religion and state.

Many of the most important burial sites are in and around the western cities of Nara and Kyoto, both ancient capitals. The biggest, belonging to Emperor Nintoku (early fifth century), is a keyhole-shaped mound near Osaka that covers almost 500,000 square metres.