Prime Minister Prayut Chan-o-cha defended a land sale by his father, insisting that it did not favour anyone after the opposition raised the issue during the first day of the censure debate against the government.
It was the first time Gen Prayut had addressed the issue publicly.
Yutthapong Charassathian, a Pheu Thai Party MP for Maha Sarakham, claimed that the land which was sold by Gen Prayut's father to a private company in 2013 was just a fish pond.
"This is not a land plot. It is a fish pond. People who are in their right mind would not buy it. The transaction was worth 600 million baht. This is unusual,'' Mr Yutthapong said.
At the time, the land was estimated to be worth about 200 million baht, Mr Yutthapong alleged.
He also claimed that part of the money from the land sale appeared in Gen Prayut's assets declaration submitted to the National Anti-Corruption Commission.
Mr Yutthapong said that the company that bought the land, 69 Property Co Ltd, had been created just seven days before the transaction and he asked Gen Prayut to explain whether he was involved.
Mr Yutthapong also alleged that this land sale was brokered in exchange for the business group which owned the company getting a contract to manage and run the Queen Sirikit National Convention Centre for 50 years.
He also accused Gen Prayut of failing to heed the Attorney-General's warning that a renewal of the contract was forbidden under the Private Investment in State Undertakings Act.
The issue of the land sale came to light in 2014 after the military coup.
At that time, Gen Prayut told reporters to stop digging into the matter, saying the land had belonged to his family since he was a boy.
Col Prapat Chan-o-cha, Gen Prayut's father, sold nine plots totalling 50 rai in Bangkok's Bang Bon district to 69 Property Co Ltd on May 9, 2013, seven days after the company was registered, according to Isara News Agency, which quoted official documents.
A major shareholder of the company at the time had a British Virgin Islands address, Isara News said, citing Business Development Department data.
The major shareholder now is a company linked to a liquor and property billionaire.
In response, Gen Prayut dismissed Mr Yutthapong's claim that the land was a fish pond.
He said the 50-rai plot is the largest in the area, with a road and a canal nearby, not a fish pond.
"Do you mean I made a deal that would reward the buyers in the future if they bought the land? How should I know whether I would become prime minister?" the prime minister asked rhetorically.
The market price of the land in 2013 was about 609 million baht. Currently, it is estimated to be worth 812 million baht, Gen Prayut said.
Finance Minister Uttama Savanayana explained that the contract to manage the Queen Sirikit National Convention Centre was renewed after the Yingluck Shinawatra government amended the act in 2013 to allow the government to review contracts with private companies.
Mr Uttama said NCC Management & Development Co, the manager of the convention centre, proposed that the contract be renewed, and Thammasat University was then commissioned to study the proposal.
The study suggested that the break-even period would be 47 years, so the 50-year-lease was appropriate, Mr Uttama said, adding the state would receive 18 billion baht in returns.
The property will belong to the state, after the lease expires or the contract is terminated, he said.
During the censure debate, Sompong Amornwiwat, the leader of the opposition and the Pheu Thai Party, outlined allegations against the government, including its failure to uphold democracy, exploiting the justice system to favour themselves and associates, failure to improve the economy and failure to suppress corruption.
Pitha Limcharoenrat, a list MP for the now-dissolved Future Forward Party, criticised Gen Prayut for failing to improve the economy. He alleged that the government's economic policies favour big businesses at the expense of the general public.
Meanwhile, nine MPs of the dissolved FFP on Monday registered as MPs of the coalition Bhumjaithai Party, a Bhumjaithai source said, adding that another six MPs from the FFP would also join the party shortly.