TOKYO - The former chief of ultra-nationalist education operator Moritomo Gakuen has insisted that Prime Minister Shinzo Abe's wife Akie had praised a plot of state land in Osaka as "good" and told him to "please push ahead" on a deal.

Yasunori Kagoike, who is facing fraud charges in Osaka, reportedly told opposition lawmakers: "There was 'no mistake' that she said something along those lines and that she received reports from me regarding negotiations to purchase the land."

Kagoike's claims, made on Friday (March 23) to opposition lawmakers who were granted access by a district judge to interview him while under custody, will likely heighten calls for Mrs Abe to answer questions in Parliament on her role behind the murky land deal.

He also alleged that officials from the Finance Ministry's Kinki Financial Bureau had taken "special interest" in the talks because it involved the Prime Minister's wife.

The raging firestorm has shaken Tokyo's political epicentre Nagatacho, where sizeable protests have broken out almost daily, and threatened Mr Abe's reign in office.

At the crux of the issue is a sweetheart deal, in which state land was sold to Moritomo Gakuen at one-seventh of its appraised value to build an elementary school, with Mrs Abe to be named honorary principal.

The Finance Ministry confessed on March 12 that its Financial Bureau had tampered with 14 documents related to the deal to remove all mention of Mr and Mrs Abe.

In some cases, sentences and even entire paragraphs were scrubbed, including one in which Kagoike was quoted as telling officials that Mrs Abe had told him: "This is a good plot of land, so please move forward."

With the scandal now effectively a he-said-she-said battle, opposition lawmakers are stepping up calls for key players in the case to speak under oath in Parliament, or the Diet. A false testimony would amount to perjury, which is a criminal offence.

Former National Tax Agency chief Nobuhisa Sagawa - who had been heading the Financial Bureau when the alterations were made between February and April last year - has been summoned to testify in the Diet next Tuesday (March 27).

Mr Abe reported to Parliament on Monday (March 19) that he had asked Mrs Abe about the purported claim, and that she had told him that she said "nothing of the sort".

He has said that he would resign as Prime Minister and lawmaker if either he or his wife were found to have been directly involved in the shady deal.

The LDP has held firm that Mrs Abe should not have to testify in the Diet, because she is a "private citizen" who has "nothing to do" with the ongoing probe into why records had been doctored.

But Kibo no To (Party of Hope) lawmaker Masato Imai told reporters on Friday: "Because we don't know to what extent Kagoike's comments are true, it is necessary to question Akie in the Diet."

The opposition may also step up calls for Mrs Abe's former secretary Saeko Tani to testify in the Diet, though it remains unclear if the LDP would relent.

The opposition has had a track record of boycotting parliamentary sessions to protest the LDP, which may cause a stalemate in policy debates.

Yet within the LDP there are concerns that Mrs Abe's free-wheeling ways might come back to bite her husband. Political observers have said that the LDP may be hesitant to summon her on the stand out of fear that she may cave under intense opposition questioning.

For now, her silence on social media speaks volumes. Previously active on Facebook and Instagram, she has been keeping mum since March 11, the eve of the Finance Ministry's confession over the altered documents.

"This time she has to watch herself, for sure," a source close to the government was quoted as saying in a Yomiuri Shimbun report on Thursday.

LDP chief deputy secretary-general Masahiko Shibayama told public broadcaster NHK on Friday that despite Mr Kagoike's claims, there was "no point" in summoning Mrs Abe to the Diet.

"The attributions to Mrs Abe in the original pre-altered documents were only the assertions by Kagoike, and I don't think whatever he says now should affect Diet deliberations," Mr Shibayama told the NHK.

"There has been no evidence whatsoever that Mrs Abe had put undue pressure for the deal, and so there is no need for Mrs Abe to testify in the Diet."

He added: "It is very doubtful how useful the one-sided comments by Kagoike, who has been detained on fraud charges, will be to shine a light on the truth."