The Pheu Thai Party has threatened to seek a Constitutional Court ruling as parliament votes in support of a method using 500 to calculate party-list seats.
The question splitting lawmakers is whether 100 or 500 should be used to calculate the number of list MPs in the next polls.
The figure 100 derives from the total number of party-list MPs while 500 would include all of the constituency MPs as well.
Under the amended constitution, there would be 400 constituency MPs in the Lower House, up from the current 350, and 100 party-list MPs, down from 150, in the next poll.
While legal experts and some MPs and senators argue against adopting the larger number, smaller parties and a growing number of legislators have stood firm that this should be used.
Small parties would find it easier to win a list seat using that method. Based on voting figures in the 2019 general election, parties would have needed around 70,000 votes to win a list seat under that model.
However, if 100 were used instead, a party would have had to capture at least 350,000 votes.
The main opposition Pheu Thai Party has been a staunch supporter of the smaller number as it is confident of capturing many of the 100 seats up for grabs in the party-list system. Other large parties including the ruling Palang Pracharath Party (PPRP) and the Democrat Party, a coalition member, were also previously backing the use of 100 seats.
However, in the latest development, coalition parties have made an about-turn and agreed to support the use of 500 instead, with coalition MPs to be allowed a free vote on the matter, government sources said.
The move came after the use of 500 received the green light from Prime Minister Prayut Chan-o-cha in what is seen as a bid to prevent Pheu Thai from winning a landslide in the next poll, the sources said.
Deputy Pheu Thai leader and chief opposition whip Sutin Klungsang said on Wednesday that if the coalition parties vote for the larger number, that is because they fear the smaller number would give Pheu Thai an advantage in the election. "They base their ideas on who will have the advantage. If 500 is used, Pheu Thai will be at a disadvantage. If 100 is adopted, Pheu Thai will have an advantage," he said. "Gen Prayut also based his order on this premise," Mr Sutin said.
"They failed to take into account a decent system of governing and did not consider whether this will be constitutional," Mr Sutin said.
If the larger number is adopted and included in the organic bill on the election of MPs, the matter will be taken to court, Mr Sutin said.
Some reports said Gen Prayut on Tuesday discussed the issue with Deputy Prime Minister and PPRP leader Prawit Wongsuwon and they agreed that 500 would be adopted. They gave coalition party leaders and senators the go-ahead to support it, the sources said. Also on Tuesday, the Democrats and Bhumjaithai agreed to allow their MPs a free vote on the matter.
Deputy Prime Minister Wissanu Krea-ngam said on Wednesday that if the matter is taken to court, the enactment of the bill on the election of MPs would have to be delayed pending the court's ruling.
The court will need at least a month to consider the matter, Mr Wissanu said, adding that if an election cannot be held as planned, the Election Commission can postpone it. However, deputy PPRP leader Chaiwut Thanakamanusorn denied the report that Gen Prayut had given his blessing for the use of 500.
"The prime minister has insisted the matter should be left to parliament to decide while Gen Prawit did not say which calculation method he preferred. It is up to the government whip and parties to discuss the issue," said Mr Chaiwut.
Late Wednesday night the parliament voted for the use of the 500 method with 354 votes in favour, 162 against, 37 abstentions and four no-votes.
The parliament rejected the calculation method in which all votes for list MP candidates will be divided by 100. The choice was turned down by 392 votes to 160 with 23 abstentions and two no-votes.