The military-backed party of Thailand's outgoing prime minister and former coup leader said Thursday it will join an opposition-led coalition, as the kingdom's post-election deadlock appears to be nearing an end.

The United Thai Nation (UTN) party said it would add its 36 seats to the multi-party partnership formed by Pheu Thai, with a vote to choose a new premier expected on Tuesday.

Prime Minister Prayut Chan-o-Cha ran as UTN's lead candidate in the May election, less than a decade after he seized power in a coup that ousted a democratically elected Pheu Thai government.

"We want the country to move forward so UTN will join the Pheu Thai government," party spokesman Akaradej Wongpitakroj told reporters after talks.

"UTN is ready to support a candidate from PT by voting for the candidate on August 22."

Voters roundly rejected army-linked parties at the ballot box, with the reformist Move Forward Party (MFP) riding support from young and urban Thais yearning for change to win most seats.

But MFP leader Pita Limjaroenrat's bid to become prime minister was thwarted by conservative senators opposed to his determination to reform royal insult laws and tackle big business monopolies.

Pheu Thai, the party associated with exiled former premier Thaksin Shinawatra, finished second in the polls and has taken the lead in coalition efforts since MFP dropped out.

As well as UTN, the coalition also includes Bhumjaithai -- another party from the outgoing army-backed government -- leading some to question how different the new government will be from the old one.

Pheu Thai will nominate business tycoon Srettha Thavisin for prime minister and the coalition looks on course to have enough votes to get him through.

To become prime minister, a candidate must be approved by a majority of both houses of parliament -- the 500 elected MPs and the 250 senators appointed under the last junta.

Former army chief Prayut has said he is leaving politics.

AFP