Thailand is to hold a third round of elections on Saturday to try to resolve the country's political crisis.
Sunday's by-elections were held in 40 constituencies
The move, announced by the Election Commission, comes after by-elections on Sunday that left at least 13 parliamentary seats unfilled.
Under the constitution, all seats must be filled by the end of this month.
The crisis that has engulfed the country has already prompted Thaksin Shinawatra to step down from his role as prime minister.
The by-elections held on Sunday aimed to fill the 40 seats left empty after the 2 April snap general election, which Mr Thaksin called in order to address mounting pressure against him.
Mr Thaksin's Thai Rak Thai party won those elections, but his victory was undermined due to a boycott by the main opposition parties and mass street protests.
Just two days after the poll, he said he would step down.
But his announcement still left the country with a major problem.
Because of the opposition boycott, many Thai Rak Thai candidates ran unopposed in the 2 April poll - which meant that, under Thai law, they had to win 20% of the vote to take the seat.
Some of these single candidates did not achieve anything close to that, especially in Bangkok and the mainly Muslim south, where opposition to Mr Thaksin is high.
In the weekend's by-election, the situation was repeated in at least 13 of the 40 unfilled constituencies, according to unofficial figures.
The Election Commission now plans to hold a new round of by-elections in these places.
The commission earlier admitted that it was unsure of its next move.
In all its 336 clauses, the Thai constitution offers no answer to the country's current dilemma.
Critics say Thaksin wants to wield influence from behind the scenes
The nine-year-old charter specifies that parliament must meet within 30 days of a general election to form a new government, and that every one of the 500 seats has to be filled.
According to the BBC's South-East Asia correspondent, Jonathan Head, there now seems little likelihood of that happening by the deadline - at the end of this month.
The continuing opposition boycott means many candidates will again run unopposed on Saturday and again fail to get the required 20% of the vote.
Election officials are considering asking parliament to convene regardless of whether all the seats are filled.
But this strategy is sure to provoke further anti-government demonstrations in Bangkok, because the opposition boycott means Thai Rak Thai would then completely dominate the next parliament.
Thailand is a country still deeply divided, our correspondent says.
Mr Thaksin's supporters in the countryside believe he has been unfairly robbed of a third election victory.
But his urban and southern Muslim opponents, who fear he is still pulling strings behind the scenes, insist he should leave the political stage altogether.