A government panel on Imperial succession agreed Wednesday to start discussing details with the possibility of having a reigning empress in mind, citing "concerns" about whether Japan can ensure a stable succession under the current male heir-only rule.

"Our conclusion is that deep concerns will remain if we maintain the tradition of the male-line Imperial succession," Hiroyuki Yoshikawa, a former University of Tokyo president who heads the 10-member panel, told reporters after its 11th meeting at the prime minister's office.

The number of those qualified to ascend the throne has become limited, given the restrictions under the current Imperial House Law and Japan's declining birthrate, he said.

"Even if we have 10 males who have an emperor as fathers, the number will become one-10th in several generations from a statistical standpoint," he reckoned.

But Yoshikawa said that stability alone will not be the issue for making the final decision and that the panel will further discuss the order of succession and who should be considered as Imperial family members.

It was the first panel meeting since it issued an interim report in late July in which it presented two options -- allowing female monarchs by revising the Imperial House Law or maintaining the current rule by allowing male members of former Imperial branch families to return to monarchy status through adoption or marriage.

Under the current Imperial House Law, only male heirs who have emperors on their fathers' side can reign.

Although the advisory panel to Prime Minister Junichiro Koizumi was expecting to see a public response on the interim report, which was reported by the media and also revealed on Koizumi's Web site, Yoshikawa claimed the panel received only about 20 opinions.