The name of Japanese Prime Minister Shinzo Abe's wife has been removed from documents regarding a suspected cronyism scandal, media say, as pressure mounts on the premier and his ally Finance Minister Taro Aso over a possible cover-up.

Key points: A finance ministry official said 14 items had been altered in documents relating to the scandal

A finance ministry official said 14 items had been altered in documents relating to the scandal The opposition is calling for Finance Minister Taro Aso to resign

The opposition is calling for Finance Minister Taro Aso to resign Mr Abe's ratings have slipped because of the suspected cronyism

Mr Abe, now in his sixth year in office, had tried to put behind him questions over the sale of state-owned land at a huge discount to a school operator with ties to his wife, Akie.

The issue last year sharply eroded Mr Abe's popularity.

His ratings rebounded thereafter, but doubts over Mr Abe and his Cabinet have been revived with a series of fresh revelations.

Abe denies favours, school operator chief arrested

Mr Abe has repeatedly denied he or his wife did favours for school operator Moritomo Gakuen, which bought the land, and has said he would resign if evidence was found that they had.

Former Moritomo Gakeun head Yasunori Kagoike and his wife were arrested in July on suspicion of illegally receiving subsidies.

Suspicions of a cover-up could slash Mr Abe's ratings and dash his hopes of a third term as leader of the ruling Liberal Democratic Party (LDP).

Mr Abe (second from right) is in his sixth year in office. ( AP: Eugene Hoshiko )

Victory in the LDP September leadership vote would put him on track to become Japan's longest-serving premier.

"The involvement of Mrs Abe has deepened. This has entered the stage where the responsibility of the Prime Minister himself will be called into question," Opposition Democratic Party leader Yuichiro Tamaki, was quoted saying by Kyodo news agency.

Documents altered at finance division's request: official

The doubts have also sparked calls for Mr Aso to quit, but the Finance Minister said on Friday he would not step down.

A finance ministry official said 14 items had been altered in the documents after February, when the scandal broke, at the instruction of the ministry's finance division to match testimony in Parliament.

Chief Cabinet Secretary Yoshihide Suga did not directly respond to a question by reporters about Mrs Abe but said Mr Aso should investigate the facts.

Mr Abe agreed the Finance Minister should take responsibility for the investigation.

"I would like the finance ministry to make all efforts to rebuild the organisation so we can regain trust and prevent such a thing from happening again," Mr Abe said.

Opposition politicians have called for Mr Aso to resign.

Mr Abe said he was deeply apologetic — but backed his Finance Minister.

"This is a situation in which the whole Government could lose trust," Mr Abe said.

"We will carry out the investigation to make the whole picture clear."

Sorry, this video has expired ABC correspondent Jake Sturmer says Finance Minister Taro Aso could be forced to resign by the end of the week. ( The World, Reuters: Toru Hanai )

Finance Minister also in hot water

The backing of Mr Aso, 77, who doubles as deputy premier, is vital to Mr Abe's bid for a third term and a key factor in the stability of his administration.

"At the very least, it seems that Mr Aso's chances of surviving as Finance Minister are diminishing rapidly," Tobias Harris, vice president of consultancy Teneo Intelligence, wrote in an email.

On Friday, National Tax Agency chief Nobuhisa Sagawa abruptly resigned over his remarks in Parliament about the case.

Mr Sagawa headed the ministry division that submitted the documents before he was tapped as tax agency chief in July, an appointment critics saw as a reward for his efforts to diffuse the issue with his statements to Parliament last year.

Some LDP members said the saga could undermine the party.

"It is inconceivable that the bureaucrats on the spot had such authority [to alter the documents]," media quoted Shigeru Ishiba, an LDP lawmaker who has made no secret of his desire to challenge Mr Abe in the party race, as saying on the weekend.

"If we don't make clear who did this, trust in the LDP will waver."

Reuters/ABC