The Move Forward Party (MFP) has given assurances that its government's policies would not harm the constitutional monarchy.
Party leader, Pita Limjaroenrat, also said on Monday that moves to amend the lese majeste law will continue, but the party will be alone in the task.
An eight-party alliance led by the MFP has signed a pact outlining the working agenda of their proposed coalition government. The 23-point memorandum of understanding (MoU) signed by the MFP, Pheu Thai, Prachachart, Thai Sang Thai, Seri Ruam Thai, Fair, Palang Sangkhom Mai and Pheu Thai Ruam Phalang parties omitted amending Section 112, known as the lese majeste law.
Among the key items, however, is a charter rewrite, reinstatement of cannabis as a narcotic drug, replacing military conscription with voluntary recruitment, and pushing for a same-sex marriage law without forcing compliance by people who consider it against their religion.
As he announced the completion of their agreement, Mr Pita said it was a historic day because it marked a peaceful transition in Thai politics. The signing was arranged to coincide with the ninth anniversary of the May 22, 2014 coup.
"The purpose of the MoU is to gather common agendas that we agree to push through government and parliament. It's our collective responsibility," he said at the press conference.
Mr Pita said every party agreed that the core policies the coalition members all agreed to back must not affect the democratic system with the king as head of state, and the revered status of the king cannot be violated.
The eight parties, with 313 House seats, on Monday met for two hours at the Conrad Bangkok, the venue of the MoU signing, before the ceremony, which was scheduled at 4.30pm.
A source close to the talks said that the MoU was revised at the request of coalition partners, including Pheu Thai, before the signing to include the phrase "the missions of the MFP-led government must not affect the democratic system with the king as head of state and the revered status of the king who cannot be violated".
According to the source, the signed MoU slightly differed from the version seen by the media. The item on "administration of justice for cases involving expressions of political views" was dropped.
The source said Pheu Thai reportedly disagreed with an amnesty plan since the coalition talks began due to concerns about conflict of interest and the MFP eventually agreed to exclude it from the MoU.
Asked about the lese majeste law, Mr Pita said the draft law, which was submitted to the House in February 2021, would be pushed by the party, and he expected its passage.
He said that under the MoU, coalition partners could push their own agendas as long as they did not contradict the MoU, and the MFP would continue to push for all policies it had campaigned on.
The MFP leader played down speculation he would not receive support from senators, saying the working teams would hold talks with the Senate to allay any concerns senators might have over the MFP's plans.
Mr Pita brushed off a rumour that Pheu Thai was in talks with the Palang Pracharath Party (PPRP) to form a rival coalition, saying the eight parties were solid.
He said it was premature to discuss the allocation of cabinet portfolios and the House speaker post following reports that he would oversee the Defence Ministry.
Meanwhile, Wanwichit Boonprong, a political science lecturer at Rangsit University, said the MoU would be unnecessary if the MFP-led coalition does not require votes from the Senate or Mr Pita does not face a probe into his qualifications.
"The MoU is something new. It is a written agreement to prevent betrayal. It's a political tactic. As for policies, they can say what they want. But the signing of the MoU is to give the MFP confidence," he said.