A former aide to Prime Minister Shinzo Abe will appear in the Diet on Thursday to testify about a veterinary school favoritism allegation leveled at the premier, ruling and opposition party lawmakers said.

With the two sides agreeing Tadao Yanase will appear as unsworn witness, major opposition parties are expected to resume attending Diet sessions which they have boycotted since mid-April. The former Abe secretary is likely to correct his earlier remarks concerning a veterinary department project by a close friend of the prime minister, a source close to the matter has said.

Hiroshi Moriyama, LDP's Diet affairs chief, told reporters after meeting Kiyomi Tsujimoto, his counterpart within the main opposition Constitutional Democratic Party of Japan, "The Diet will completely return to normal" from Tuesday.

Opposition parties have boycotted parliamentary deliberations since mid-April to protest the government's handling of the veterinary school matter and of other scandals.

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

Yanase, who currently serves as vice minister for international affairs at the Ministry of Economy, Trade and Industry, is alleged to have recommended, while a secretary to Abe, that local government officials move forward with the plan to build the school by saying it was a "matter concerning the prime minister."

In previous testimony before the Diet, Yanase said he does not remember meeting local officials over the school issue. He is now expected to admit he met with local officials at the prime minister's office on April 2, 2015, while continuing to assert he never referred to the project as "the prime minister's matter," the source said.

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 new veterinary department in Japan in half a century.