The Pheu Thai (PT) Party is confident its prime ministerial candidate will win endorsement from parliament in the next PM vote, a party source said.

However, the source said that before the vote, things may take an unexpected turn, and Palang Pracharath Party (PPRP) leader and PM candidate Prawit Wongsuwon could emerge as the new prime minister, with the support of renegade MPs from Pheu Thai.

The source said that all eyes are on whether parliament president Wan Muhamad Noor Matha will today issue a letter setting a date for the next PM vote. The voting session must be set three days in advance.

If the vote is to be held on Friday, the source said the letter must be issued by today.

The source went on to say that a team of Pheu Thai negotiators have now gathered enough support for its PM candidate, Srettha Thavinsin, from other parties.

The source said the senators are also likely to vote in his favour as Pheu Thai has turned its back on the Move Forward Party (MFP).

But if the senators still refuse to vote for Mr Srettha, this means Pheu Thai is being used as a springboard for Prawit to become the next prime minister, the source said.

Some believe senators may not vote for any of Pheu Thai's three candidates but choose to support those from the "conservative" camp as it attempts to secure the premiership.

Anutin Charnvirakul, leader and PM candidate of the Bhumjaithai Party, or Prawit, may benefit from this situation.

The other two Pheu Thai PM candidates are Paetongtarn Shinawatra, daughter of former PM Thaksin Shinawatra, and Chaikasem Nitisiri.

"Pheu Thai executives must make a wise decision as the stakes are high. During the campaign for the May 14 election, we promised that we would not work with the 'uncle' parties [referring to parties linked with military leaders involved in the 2014 coup]," the source said.

The "uncles" refer to Prime Minister Prayut Chan-o-cha and Prawit.