Pokin Polakul, Watana Muangsook and Pongsakorn Annanopporn – non-MP members of the party – quit alongside her. More members are likely to follow suit in the coming days.
Khunying Sudarat did not offer any explanation for her decision in her resignation letter to the party’s registrar.
The 59-year-old politician resigned as the party’s chief strategist in late September.
She had been a key Pheu Thai member, since it was born following the disbanding of Thai Rak Thai, which she co-founded in 1998.
Second on Pheu Thai’s party list at the 2019 election, she failed to get into parliament when the party failed to win any party-list seats and was one of the party’s three prime ministerial candidates, along with Chadchart Sittipunt and Chaikasem Nitisiri.
Meanwhile, the Democrat Party has decided not to support a motion seeking a Constitutional Court ruling on charter amendment bills that will revise Section 256 of the current charter to pave way for the setting up of a charter drafting body.
Democrat leader Jurin Laksanavisit said the party MPs agreed to reject the motion because the charter amendment drafts were approved in the first reading by parliament.
He said if parliament has questions concerning the legality of the charter amendment bills, it can seek a Constitutional Court ruling after the third and final reading, before submission for royal approval.
Mr Jurin said that if the drafts are forwarded to the Constitutional Court for a ruling, it will give the impression that the government wanted to stall the amendment process.
The motion was signed by 47 senators and 25 MPs, all from the main coalition party, Palang Pracharath, who claimed parliament was not authorised to “make” a new charter.
Any action to allow the writing of a new charter would be unconstitutional, according to the motion.