can be expanded to a maternal line," panel Chairman Hiroyuki Yoshikawa, a former president of University of Tokyo, told a news conference after Tuesday's panel session.
"It's almost certain that the (tradition of) paternal-line-only succession can't continue to exist," he added.
Since the birth in 1965 of Prince Akishino, Emperor Akihito's second son, no males have been born to the Imperial family, putting the survival of the throne in jeopardy.
The panel was tasked in January with discussing measures for a "stable succession," including revisions of the 1947 Imperial House Law, which stipulates that only a male in the male line can ascend to the throne.