The ruling Palang Pracharath Party (PPRP) candidate won the by-election in constituency 2 of Kamphaeng Phet, unofficial results indicated late on Sunday night.
Petchpoom Apornrat of the PPRP was said to have won 45,687 votes to secure victory while his rival, Kumpol Panyakul, from the main opposition Pheu Thai Party received 37,989 votes.
Voter turnout in the constituency was expected to fall short of that in last year's general election with several complaints raised about polling irregularities, says the Election Commission (EC).
The by-election across 196 polling stations proceeded in an orderly manner. The poll opened at 8am and closed at 5pm. Thanaboon Sinmana, director of the provincial election office, said the turnout was unlikely to reach 74% recorded in the March 24 general election last year.
He said some voters were working outside the constituency and did not return home to vote. Also, there was no advance voting. The unofficial poll results were expected to be known later last night.
The EC called the by-election to fill the vacancy left by Waipot Apornrat, a former Palang Pracharath Party (PPRP) MP, who was sentenced to 50 years in jail for forest encroachment in his hometown.
Waipot also was stripped of his MP status after the Supreme Court sentenced him to life in jail on Jan 15 for his role in a riot that shut down the 2009 Asean Summit.