The Constitutional Court ruled yesterday that non-Thai citizens cannot be included as part of the population in calculating the number of people represented by each MP.
The ruling was in response to a petition lodged against the Election Commission (EC) after several politicians and academics voiced disagreement with its methodology.
The EC had included three groups of non-Thais in the population database to guide the redrawing of constituency boundaries.
Among them were permanent residence permit holders, those with temporary residences and individuals who have been living in Thailand for more than ten years. However, migrant workers from neighbouring countries were not included in the poll agency's electoral calculation.
The court ruling, which does not affect previous elections, means the EC has to recalculate the boundary lines.
A source at the EC said yesterday the poll agency had a contingency plan, and provincial election offices are ready to recalculate following the court's decision.
According to one source, based on the court ruling, there will be one member of parliament per 162,766 members of the public, if calculated assuming a Thai population of 65,106,481, as recorded on Dec 31 last year.
It is speculated that the number of constituency MPs in eight provinces will change: Tak, Chiang Rai, Chiang Mai, and Samut Sakhon will have one fewer MP each, while Udon Thani, Lop Buri, Nakhon Si Thammarat and Pattani will be allocated one more apiece.
The change in the number of MPs is said to affect the election odds of the Bhumjaithai Party the most, especially in Tak and Samut Sakhon, as Pheu Thai, the Democrats, United Thai Nation and the Palang Pracharath Party (PPRP) may now get more seats.
It is expected that the number of House seats in the South will likely increase to 60 from 58 while that in the northeastern region will increase from 132 to 133.
House seats in the northern region will shrink to 36 from 39, while the number of seats in the central, eastern and western regions will remain unchanged.
Paiboon Nititawan, deputy leader of the PPRP, said the ruling has cleared any doubts and removed any conflicts that might arise from the calculation method.
He said the ruling party is ready for the polls. Still, it will have to wait for the electoral map before it submits its list of constituency candidates to party representatives for their opinions.
Mr Paiboon said the process is in line with the law governing the affairs of political parties and insisted that PPRP leader Gen Prawit Wongsuwon is the party's sole candidate for the prime minister post.
Nikorn Chamnong, a list-MP of the Chartthaipattana Party, said political parties could not finalise the selection of election candidates until the EC publishes an electoral map.