The Election Commission (EC) has come under fire for what has been labelled its "bizarre" incorporation of non-Thai people in the population used for redrawing the constituency boundaries for the next poll.
Some parties also say the EC has disadvantaged them by making particular constituencies smaller.
The EC secretary-general, Sawang Boonmee, said three groups of non-Thai people are being included in the population database used for redrawing the constituency boundaries as dictated by the law.
However, the groups exclude migrant workers from neighbouring countries.
Mr Sawang insisted the inclusion of the non-Thais was required by the inclusion of the term rassadon (citizens) in the Interior Ministry's central registry regulations.
The regulation requires an update of the population to be made on Dec 31 of every year with the number to be reflected officially in March the following year.
"Rassadon" includes both Thai citizens and non-Thais who fall into three groups: people with permanent residence, those with temporary residence and also individuals who have been living in Thailand for more than 10 years.
The definition also covers ethnic minority groups whose problems with citizenship status are pending rectification by the government.
The non-Thais counted in the population database must be eligible for listing in the domicile registration under Section 29 of the Citizen Registry Act, according to the EC.
Rangsiman Rome, spokesman for the Move Forward Party (MFP), slammed the methodology as "bizarre".
"The easy question that the EC must answer is: Will these non-Thais be able to vote?
"If not, they should not have been combined in the population pool in the first place," he said.
Although the segment may be too small to make a difference in the polls, there is no justification for the inclusion of non-Thais in the population database, he said.
Mr Rangsiman said he was concerned that if the method went uncorrected, it may undermine the legality of the next general election and complaints could even see the result overturned.
"It would be a major setback for society. Nothing should stand in the way of a free and fair general election," he said.