As speaker, Yongyuth's first task is to call for parliament to convene and select the country's 25th prime minister, which is expected to be on Friday.
Samak Sundaravej, leader of the PPP, is expected to be nominated and voted as the prime minister by the coalition government led by the PPP.
Yongyuth's election was criticised by members of the opposition and democracy activists who say the election commission is still investigating vote-buying allegations against him.
He, however, said he was confident of clearing himself of the "groundless accusations".
Major repercussions
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"It is ridiculous to appoint a person who has a tainted record of election fraud to be chief of the legislature"
Varin Thiemjaras, People's Network for Elections |
Somchai Juengprasert, the election commissioner, said they will pursue the case carefully because an unfavourable ruling could have major repercussions.
A guilty verdict against Yongyuth could lead to the dissolution of the PPP, which in turn would lead to the dissolution of government.
Varin Thiemjaras, an activist with People's Network for Elections, said: "It is ridiculous to appoint a person who has a tainted record of election fraud to be chief of the legislature."
On Monday, Thailand's first elected parliament sat for the first time since the military coup 16 months ago, with the PPP dominating the house with 233 of the 480 seats.
Crown Prince Maha Vajiralongkorn, who represented 80-year-old King Bhumibol Adulyadej at the opening of parliament, urged a return to stability and national unity.
The PPP government is also expected to welcome Thaksin back to Thailand from exile by April.
Despite facing fraud charges in Thailand, he remains popular among the rural majority who benefited from his populist policies and voted heavily in favour of his allies in the PPP in last month's polls.