Japan's Finance Ministry acknowledged on Monday that it doctored documents in a widening scandal linked to Prime Minister Shinzo Abe's wife that has rattled his government and caused its support ratings to slide.

Abe quickly apologized on Monday on behalf of ministry officials but did not mention his wife or her suspected role in the scandal.

"People are looking critically at the developments, and I take it seriously," he said, promising to pursue a thorough investigation into what caused the problem.

The altered documents relate to the 2016 sale of state land to school operator Moritomo Gakuen in Osaka at one-seventh of the appraised value with the alleged involvement of first lady Akie Abe, who supported the school's ultra-nationalistic education policy.

An investigation by the ministry showed that the school operator told officials that Akie Abe encouraged him to proceed with the land deal, and several conservative lawmakers had contacted the ministry about the school plan, but it was not clear whether they violated any law.

It said one document originally noted that the school operator was involved with a powerful pro-Abe political lobby, Nippon Kaigi, of which Abe was vice chairman, but that comment had later been deleted.

The scandal, which surfaced a year ago, has smoldered despite a major election victory by Abe in July as opposition lawmakers continued to scrutinize the case. It erupted again in recent weeks after a major newspaper reported that it found evidence the ministry had altered records after the scandal broke.