BANGKOK: Most Thais disagree with the leading plan for a coalition government which includes military-backed groups, an opinion poll showed on Sunday, two days before a parliamentary vote aiming to end a three-month political stalemate.

About 64% of 1,310 respondents disagreed or totally disagreed with the idea of the Pheu Thai party forming a "special government" with military-backed rivals, according to the survey by the National Institute of Development Administration.

Thailand has been under a caretaker government for five months and faces prolonged uncertainty after the winner of the May election, Move Forward, was blocked from forming a government by conservative legislators allied with the royalist military.

The second-place Pheu Thai, founded by the family of self-exiled billionaire former Prime Minister Thaksin Shinawatra, this month took over efforts to form a government.

Paetongtarn Shinawatra, Thaksin's daughter and one of three prime ministerial candidates of Pheu Thai, on Sunday apologised that the party had failed to keep its election pledge of not joining with pro-military parties.

"We have to make adjustments to keep the country going," she told reporters. "Of course, Pheu Thai has the price to pay, that is the criticism of the people. We humbly accept and apologise for making many disappointed and sad."

The party will work fully to solve the country's problems if it can form a government, Paetongtarn added.

Pheu Thai, set to nominate another candidate, real estate tycoon Srettha Thavisin, as prime minister for voting on Tuesday, needs the support of more than half the bicameral legislature, including the military-appointed Senate.

Also on Tuesday, Thaksin is set to return to Thailand, despite facing a jail sentence, Paetongtarn said on Saturday.

On Sunday, she said Thaksin's return had nothing to do with politics and he simply wanted to return to his home country.

Pheu Thai governments were ousted by military coups in 2006 and 2014 - which ousted Thaksin and his sister Yingluck Shinawatra, respectively - when the party's interests clashed with the country's powerful old money elites and royalist military.

Sunday's poll found Paetongtarn would be the preferred prime minister with 38.6% support, followed by Srettha at 36.6%.

Pheu Thai on Thursday gained support from the military-backed rival United Thai Nation Party. A lawmaker from another pro-military party, Palang Pracharat, said this month the party would back Pheu Thai in trying to break the protracted deadlock.