A former aide to Prime Minister Shinzo Abe is planning to correct his earlier remarks to the Diet over a veterinary school project at the center of favoritism allegations leveled at the premier, a ruling bloc source said Wednesday.

The ruling parties are considering summoning Tadao Yanase, who served as Abe's secretary, to the Diet possibly next week as an unsworn witness, in an attempt to bring back opposition lawmakers who have been boycotting parliament deliberations, the source said.

The opposition parties have refused to join deliberations since mid-April in protest at the government's handling of the matter and a number of other scandals while demanding Yanase appear in the Diet as a sworn witness, who can be punished if the person provides false testimony or refuses to testify without appropriate grounds.

The main opposition parties plan to discuss Monday whether they should return to Diet talks following Japan's Golden Week holiday period.

Kake Educational Institution, a school operator managed by Abe's close friend Kotaro Kake, last month opened the veterinary department at its Okayama University of Science in a specially deregulated zone in Imabari, Ehime Prefecture.

(Yanase speaks in a parliamentary session in front of Abe and Finance Minister Aso on July 24, 2017)

Yanase, who currently serves as vice minister for international affairs at the Ministry of Economy, Trade and Industry, is alleged to have recommended that local government officials go ahead with the plan by saying it was a "matter concerning the prime minister," a charge he has denied.

"As far as I can remember, I haven't met (the local officials)," Yanase told the Diet. But he is now expected to revise his remarks while admitting that he met the school operator officials in the prime minister's office on April 2, 2015, according to the source.

But the senior bureaucrat is likely to deny his reported remark that the school project is "the prime minister's matter," the source added.

Yanase's expected change of story is likely to deepen suspicion that Abe used his influence in the approval process that concluded with the setting up of the first veterinary department in Japan in half a century.

The meeting drew attention last month after an Ehime prefectural government document was found that recorded a Tokyo visit by officials of the prefectural and city governments as well as the school operator.

Another document almost identical to the one found in Ehime was discovered in the farm ministry, and an email in education ministry records also suggested the gathering took place. Both ministries were involved in the school project.

The revelations have made it difficult for the ruling camp to continue to rebuff opposition party calls for Yanase to make another appearance in parliament, the source said.