Prime minister Shinzo Abe and finance minister Taro Aso have been under pressure over the suspected cover-up of a cronyism scandal after it emerged that officials had falsified documents related to a state land sale last year.

Abe denied that either he or his wife Akie had intervened in the sale or were involved in altering documents related to the deal, in which mention of his and his wife’s names were removed.

The survey, covered in the liberal Asahi newspaper, found 48% of Japanese people polled said Abe and his government should quit, compared to 39% who said it was not necessary.

Abe’s support slid to 32.6%, down 11.7 percentage points compared to last month, the survey found. The number of people who said they did not support the prime minister rose 13.2 percentage points to 54.9%.

The prime minister told parliament last year when the scandal broke that he would resign if any link emerged between himself or his wife over a property deal in which a school bought public land at a heavy discount.

The scandal first emerged last year when Asahi reported that educational foundation Moritomo Gakuen, with alleged ties to Abe and his wife, bought government land for a fraction of the price of comparable plots.

Abe is Japan’s longest serving prime minister, retaking office in 2012 with the promise to reboot the economy and defence. He also served as prime minister from 2006-07.

Finance minister Aso said on Monday he would not resign. But he told reporters: “It could shake confidence in the administration as a whole. I strongly feel responsibility as the head of administration.”

At a separate news conference he said a number of officials at his ministry in charge of the sale had altered the documents.

A finance official said that 14 items had been altered in the documents after February last year at the instruction of the ministry’s financial division in order to match parliamentary testimony.