Srettha Thavisin cannot serve as a caretaker prime minister if he is removed from office on Wednesday by the Constitutional Court over his controversial decision to appoint ex-convict Pichit Chuenban as a cabinet minister, Wissanu Krea-ngam, the PM's adviser, said.
If the court decides to remove Mr Srettha from office, the cabinet will also vacate office automatically as a result, said Mr Wissanu, a former deputy prime minister and a legal expert.
If Mr Srettha loses his job, Deputy Prime Minister and Commerce Minister Phumtham Wechayachai will take over as caretaker prime minister, Mr Wissanu said.
The comment differed from a reported statement earlier, in which he indicated Mr Srettha could assume a caretaker prime minister role if he is removed from office.
Asked if Mr Srettha, who is one of Pheu Thai's three prime ministerial candidates, could be re-nominated as prime minister if the Constitutional Court rules against him, Mr Wissanu said: "I believe he could. But we have to wait for the court's ruling."
Mr Wissanu also said that a new prime minister must come from the lists of prime ministerial candidates submitted by parties to the Election Commission ahead of last year's election.
A candidate must have the support of more than 25 MPs to be nominated in parliament.
Mr Wissanu added a non-MP cannot be nominated for prime minister because the coup-appointed Senate, which had the power to co-elect a PM under Section 272 of the constitution, no longer exists after its five-year term ended in May. The new Senate does not have the power to co-elect.
Under the constitution, if a joint sitting of parliament fails to select a new premier from party candidate lists, Section 272 would trigger an alternative route.
In such a situation, half of the 750 MPs and senators can initiate a motion to suspend the rule requiring that PM candidates come from party lists, paving the way for an outsider to be selected.
A petition seeking to dismiss the prime minister was submitted in May by a group of 40 former senators who said Pichit should not have been considered for a cabinet post in the last cabinet reshuffle. This is because he was convicted of contempt of court for attempting to bribe Supreme Court officials in 2008 while representing former PM Thaksin Shinawatra in a controversial land case.