By Linda Sieg

TOKYO, March 12 (Reuters) - Japan's finance ministry is set to report on Monday that documents were doctored in a suspected cronyism scandal, which if true would add further pressure on Japanese Prime Minister Shinzo Abe and his close ally, Finance Minister Taro Aso.

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, behind him. His ratings had rebounded from a slump last year, but doubts have revived with a series of fresh revelations.

Suspicions of a cover-up could slash Abe's ratings and dash his hopes of a third term as ruling Liberal Democratic Party leader. Victory in the LDP September leadership vote would put him on track to become Japan's longest-serving premier.

A March 9-11 survey by the Yomiuri newspaper showed support for Abe's cabinet fell to 48 percent, down six points from a month earlier. Non-support rose to 42 percent and 80 percent said that the matter had not been handled appropriately.

A source with direct knowledge of the matter told Reuters on Saturday the finance ministry would report to parliament on Monday that it had found instances where content was deleted from documents submitted to lawmakers. The source spoke on condition of anonymity.

Media said the changes were made after February last year - when the suspected scandal broke - and that words describing the "special nature" of the deal were excised along with the names of several politicians.

"If this is true, isn't political responsibility unavoidable?" said Akira Nagatsuma, a senior lawmaker in the opposition Constitutional Democratic Party of Japan, on Sunday.

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 were found that they had. The issue last year sharply eroded Abe's popularity.

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

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

Also on Friday, media said police were investigating as possible suicide the death of a finance ministry official whose local office had handled the land sale.

"If Minister Aso signed off on National Tax Agency chief Sagawa's resignation knowing about the falsified documents, moves seeking his resignation are inevitable," Yuichiro Tamaki, leader of the opposition Democratic Party, said on Sunday.