Deputy Prime Minister Wissanu Krea-ngam on Monday rejected returning to a single-ballot election method, insisting the country had come too far to go back to the old system.
Reinstating the single-ballot method would require yet another charter amendment, said Mr Wissanu, also a noted legal expert.
"Frankly, I don't see the point of fixing the constitution again," he said, indicating also that he was in the dark over where the election switch-back speculation came from.
The deputy prime minister denied any cabinet-level talks or moves to bring back a single ballot following a wrangle over the calculation of list MPs during the process to amend the organic law on the election of MPs.
The committee scrutinising the bill had agreed to divide the voter population by 100 in calculating how many list MPs a political party will get. But lawmakers later voted to divide by 500 instead, thereby lowering the number of votes needed to win a list seat, much to the delight of small parties.
However, critics argued 100 must be adopted as the divisor since the next election will use a dual-ballot system in which voters will choose a candidate separately from a political party.
Since there will be 100 list seats up for grabs, the divisor of 100 was in order, according to critics, including the main opposition Pheu Thai Party, who maintained that designating 500 as the divisor would mean using the number of voters in both the constituency and list methods to calculate the list MPs, which is incorrect and unconstitutional.
However, a consensus over the calculation and amending of the organic law must be reached by Aug 15 or the current draft used instead.
Mr Wissanu admitted the cabinet had never mulled a Plan B. "Everyone thought the current version would be scrutinised without a hitch," he said.
Parliament has two sessions, on Tuesday and Wednesday, in which to deliberate the bill and hopefully iron out the differences, he added.
On Monday, the opposition renewed its threat to seek legal action against MPs and senators who voted for 500 as the divisor on grounds of defying the constitution.
Meanwhile, Somkid Chuakong, a Pheu Thai MP for Ubon Ratchathani, said the party plans to petition the National Anti-Corruption Commission (NACC) to consider prosecution against such lawmakers after the draft amendment to the organic law clears its third reading.
The opposition may also request a Constitutional Court ruling on the legality of the 500-divisor, he claimed.
Assuming the bill passes its third and final reading with the 500-divisor intact, it will then be passed to the EC for consideration. If the EC does not agree with its contents, the agency will send the draft back to parliament which will decide whether to seek the constitutionality ruling.
"But I'm not hopeful the parliament will do that [seeking the ruling]," he said.
In response, chief government whip Nirote Sunthornlekha slammed the opposition over its threat to refer the matter to the NACC, labelling it an act of intimidation.