Both the Pheu Thai and Democrat parties insist they will not field any candidates for the Dec 22 byelections to fill eight seats left vacant by former Democrat MPs.
Friday was the last day for candidates to register for the poll.
Pheu Thai spokesman Prompong Nopparit said on Friday that his party would not take part in the campaign.
It would be better to give smaller parties opportunities to help govern the country than for Pheu Thai to take part in the Democrat Party's "stunt show", he said.
Suthep Thaugsuban, a former Democrat deputy premier and MP for Surat Thani, is among the eight Democrats who resigned their seats last month in order to lead anti-government protests.
The Democrats are not contesting the seats because they believe the current government has lost legitimacy and the electoral process will not solve the country's problems.
Mr Prompong said the opposition party and the eight former MPs should be charged for the cost of holding the byelections, which is around 80 million baht, according to Election Commission (EC) member Sodsri Sattayatham.
Democrat spokesman Chavanond Intarakomalyasut confirmed that his party would not compete for any of the seats.
It is not appropriate for the Democrats to field any candidates because the party no longer believes that the election system is the solution to the crisis that Thailand is facing, he said.
Mr Chavanont reiterated that the only way out is for Prime Minister Yingluck Shinawatra to dissolve the parliament.
In addition to Mr Suthep, the others who resigned their seats were former Songkhla MP Thaworn Senneam, Sathit Wongnongtoey (Trang), Issara Somchai (party list), Witthaya Kaewparadai (Nakhon Si Thammarat), Chumpol Pholsai (Chumphon), and former Bangkok MPs Phutthipong Punnakan, Ekkanat Prompan and Nataphol Teepsuwan.