Prime Minister Yoshihide Suga faced backlash Friday after he decided not to appoint academics who have been critical of the nation's security and anti-conspiracy legislation to a science council that makes policy recommendations to the government.

Prime ministers have been naming members of the Science Council of Japan, an organization under the jurisdiction of the premier but operated independently from the state, as recommended by the council since 2004 based on the law on the organization.

But Suga did not name six of the 105 people recommended on Thursday and gave no specific reasons why. Chief Cabinet Secretary Katsunobu Kato told a press conference the same day the prime minister "is not obliged" to appoint the recommended people to the council.

The council's President Takaaki Kajita, a Nobel Physics prize winner in 2015 who assumed the top post just Thursday, said the organization wants to know the reasons behind the decision and have the six appointed.

"The council makes output based on studies independent from the government. Such basics should not change," Kajita told reporters.

"The refusal (to appoint) without a clear reason raises constitutional questions," said Takaaki Matsumiya, a Ritsumeikan University professor specializing in penal code, who was among the excluded six, during a joint hearing held by opposition parties.

Matsumiya is known for speaking at the Diet in 2017 against what was known as the conspiracy bill, which was aimed at thwarting terrorism but which fanned concern for the suppression of civil liberties.

Ryuichi Ozawa, a Constitution professor at Jikei University School of Medicine, also criticized the move, calling it "a major infringement of academic freedom."

Ozawa said in 2015 at the Diet that Japan's security legislation enabling Tokyo to exercise the right to collective self-defense was unconstitutional.

In addition to Matsumiya and Ozawa, the four other academics who were not named are professors Masanori Okada of Waseda University, Shigeki Uno and Yoko Kato of the University of Tokyo and Sadamichi Ashina of Kyoto University.

Kato said at a regular press conference the appointment of the council's members is under the legal jurisdiction of the prime minister and that "the government will appoint members based on its own views."

The top government spokesman declined to give the reasons that led to Suga's decision and said the premier will not reverse it. Kato added he believes the decision will not immediately affect research by the members of the council.

Still, opposition parties and many others called for a further explanation from the government on the matter.

Keiichi Ishii, secretary general of the Komeito party, the junior coalition ally of the ruling Liberal Democratic Party, urged the government to "give an explanation as thoroughly as possible" even if they cannot give the full details.

The Science Council of Japan was established in January 1949 for the purpose of promoting and enhancing the field of science, and having science reflected in government, industries and people's lives. With some 210 members and 2,000 associates, it makes policy recommendations to the government and public.