The Move Forward Party's (MFP) bid to form a governing coalition may not be easy after a number of senators made it clear they will not support its prime ministerial candidate.

Under the constitution, the 250 senators appointed by the now-defunct National Council for Peace and Order (NCPO) are allowed to join MPs in electing a prime minister.

It will be the second and last time they will co-elect a prime minister after Sunday's election. In the 2019 election, they joined MPs in voting for Gen Prayut Chan-o-cha to become prime minister.

MFP leader Pita Limjaroenrat on Monday announced a plan to form a coalition government comprising five former opposition parties and one new party, with a combined 310 MPs and himself as prime minister.

Mr Pita made the announcement after the Election Commission declared the MFP as the winner of the most seats in Sunday's general election with 152 MPs.

However, Senator Jadet Insawang said that upon taking the post of a senator, he took an oath to protect the constitutional monarchy.

"The MFP and Mr Pita once announced they would scrap Section 112 [the lese majeste law], which will affect the monarchy. This is unacceptable," he said.

"If the MFP gathers the support of 376 MPs [more than half of the 750 members of the Lower and Upper Houses,] there is no need to seek the Senate's support for [a prime ministerial candidate]. But if they only get 309, they'll have to.

"For me, if Mr Pita, the MFP's prime ministerial candidate, is nominated for a vote in parliament, he won't get my vote," Mr Jadet said.

Senator Kittisak Rattanawaraha said that the senators would look at the qualifications of any prime ministerial candidate nominated for a vote in parliament.

"One of the qualifications is that the candidate must be loyal to the country, religion and the monarchy," he said.

Another senator said that any party that wins the most seats must gather the support of 376 MPs so they can nominate a prime ministerial candidate and bypass the Senate.