As the Constitutional Court has rejected the Election Commission's (EC) petition seeking a ruling on the calculation method for party-list MPs, the poll agency could be sued by political parties that are disadvantaged by any formulas it has devised, pundits say.

Jade Donavanik, a former adviser to the Constitution Drafting Committee (CDC), told the Bangkok Post the EC had hoped the charter court would rule on the matter.

However, as the court refused to accept the petition, the EC is on its own, he said.

"The EC must step up to the task and come up with a formula that adheres to the principles contained in the charter, as well as the organic laws governing the election of MPs," said Mr Jade.

The former CDC adviser said that if the EC comes up with a formula that favours smaller political parties which had failed to meet the parliamentary threshold under the mixed-member proportional (MMP) representation system, larger parties would immediately lodge complaints against the poll agency, he said.

"They would accuse the EC of abusing its powers or even negligence," said Mr Jade, before adding that the reverse is true if the poll agency comes up with a formula that favours larger parties.

Acting deputy leader of the Democrats, Satit Pitutacha, told the Bangkok Post that from now on, the EC must tread carefully in interpreting the law to formulate its calculation method.

"The EC cannot be biased towards any political party," Mr Satit said.

Sawaeng Boonmee, deputy secretary-general of the EC, said the poll agency will have to come up with its own formula to calculate party-list MPs by May 9, as the agency is required to officially endorse and announce the result of the March 24 election on that date.

"If we can't do so, we would be breaking the law," Mr Sawaeng said.