A failure to amend the organic law on the election of MPs by the Aug 15 deadline would leave the method of calculating the list MPs in the hands of the Election Commission (EC), according to Deputy Prime Minister Wissanu Krea-ngam.
It would also mean six months of wasted political time, said Mr Wissanu, the government's top legal expert, with lawmakers so far still unable to thrash out the issue of whether list seats should be calculated by means of adopting 100 or 500 as the divisor.
Taking the number of ballots cast for a party and dividing that by 100 -- the number of list MPs up for grabs -- was originally accepted by the parliamentary committee tasked with scrutinising the draft amendment.
However, when the bill was put to a vote in parliament, a majority of lawmakers voted to use 500 as the divisor instead, prompting its return to the scrutiny panel.
The revised version has since been completed and is now pending a third and final reading.
Critics have warned of profound political complications from using 500 as the divisor.
Ruling Palang Pracharath Party (PPRP) deputy leader Paiboon Nititawan is moving to sink the adoption of 500 as divisor on the grounds it would be unconstitutional.
He said on Tuesday that he would petition the Constitutional Court to rule on the matter in his capacity as former chairman of the committee in charge of reintroducing the dual-ballot system.
Mr Paiboon argued that since the charter was being rewritten to accommodate a dual ballot, the calculation of list MPs must use the lower figure which reflects the amount of list seats up for grabs.
The alternative would require including the constituency votes which is the antithesis of the intent to separate votes for constituency MPs and parties.
"It doesn't take an expert to see that list MPs may only be obtained by calculating from the pool of ballots cast for a party and dividing that by the number of list MPs there is room for.
"Anything other than that distorts the principle and spirit of voters casting two ballots. The issue will be resolved in the Constitutional Court," he said.
"However, it is feared the divisor wrangle will see the revised bill fail to meet the Aug 15 deadline, which Mr Wissanu said would result in a fallback to the dual ballot and 100 divisor originally proposed by the EC.
Mr Wissanu said that it would be a pity if that were to happen as parliament has gone to great lengths to modify the bill.