A new legal controversy has emerged since the royal decree to open the Senate's extraordinary session did not include a vote for speaker on the agenda, said Niwatthamrong Bunsongphaisan.
The decree only allowed the Senate to choose a justice of the Administrative Court and a member of the National Anti-Corruption Commission (NACC) based on Section 132 (2) of the Constitution.
Surachai Liangboonlertchai won a 96-51 vote to become the new speaker in the election on Friday night.
Earlier on Saturday, the Pheu Thai Party's legal team led by Kanin Boonsuwan and Noppadon Pattama said the election might be illegitimate and the Constitutional Court might need to rule on the matter.
Former Senate speaker Nikom Wairatpanich expressed the same view.
Mr Nikom said that if anti-government activists want to question the legitimacy of Mr Niwatthamrong's premiership, then logically they must question the legitimacy of the Senate speaker as well.
Protesters who want to delay the July 20 election until reforms can be carried out say that as acting caretaker premier, Mr Niwatthamrong lacks the prerogative to submit the royal decree on the election to His Majesty the King for endorsement.
"If Mr Niwatthamrong cannot submit the election decree to His Majesty, he won't be able to submit the Senate speaker's appointment to the King as well," said Mr Nikom.
"Besides, the Senate speaker's election could be viewed as a conflict of interest under Section 122 because Mr Surachai is the one who called the meeting. He also acted as its chairman and debated duering the election in which he later ran.
"In any case, the new Senate speaker will not have full authority without royal endorsement," he continued. "Without it, a Senate speaker-elect could not call a meeting to perform its duties such as removing Ms Yingluck [Shinawatra], as well as 36 senators and me, who were charged by the NACC."
Mr Nikom, a former elected senator for Chachoengsao, had to cease his duties after the National Anti-Corruption Commission decided that he, along with 36 senators, had a conflict of interest when they voted for a charter amendment for a fully elected Senate.